LINER LTC1709-8 2-phase, 5-bit vid, current mode, high efficiency, synchronous step-down switching regulator Datasheet

LTC1709-8/LTC1709-9
2-Phase, 5-Bit VID,
Current Mode, High Efficiency,
Synchronous Step-Down Switching Regulators
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FEATURES
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DESCRIPTIO
Single Controller Operates Two Output Stages
Antiphase Reducing Required Input Capacitance
and Power Supply Induced Noise
Two 5-Bit Desktop VID Codes:
LTC1709-8 For VRM8.4 (VOUT from 1.3V to 3.5V)
LTC1709-9 For VRM9.0 (VOUT from 1.1V to 1.85V)
Current Mode Control Ensures Best Current Sharing
True Remote Sensing Differential Amplifier
Power Good Output Indicator
OPTI-LOOPTM Compensation Minimizes COUT
Programmable Fixed Frequency: 150kHz to 300kHz
±1% Output Voltage Accuracy
Wide VIN Range: 4V to 36V Operation
Adjustable Soft-Start Current Ramping
Internal Current Foldback and Short-Circuit Shutdown
Overvoltage Soft Latch Eliminates Nuisance Trips
Low Shutdown Current: 20µA
Available in 36-Lead Narrow SSOP Package
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APPLICATIO S
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Workstations
Internet Servers
Large Memory Arrays
DC Power Distribution Systems
An internal differential amplifier provides true remote
sensing of the regulated supply’s positive and negative
output terminals as required for high current applications.
The RUN/SS pin provides soft-start and optional timed,
short-circuit shutdown. Current foldback limits MOSFET
dissipation during short-circuit conditions when the
overcurrent latchoff is disabled. OPTI-LOOP compensation allows the transient response to be optimized for a
wide range of output capacitors and ESR values. The
LTC1709-8/LTC1709-9 implement two different VID
tables compliant with VRM8.4 and VRM9.0 respectively.
, LTC and LT are registered trademarks of Linear Technology Corporation.
OPTI-LOOP is a trademark of Linear Technology Corporation.
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The LTC®1709-8/LTC1709-9 are 2-phase, VID programmable, synchronous step-down switching regulator controllers that drive two all N-channel external power MOSFET
stages in a fixed frequency architecture. The 2-phase
controller drives its two output stages out of phase at
frequencies from 150kHz to 300kHz to minimize the RMS
ripple currents in both input and output capacitors. The 2phase technique effectively multiplies the fundamental
frequency by two, improving transient response while
operating each channel at an optimum frequency for
efficiency. Thermal design is also simplified.
TYPICAL APPLICATION
+
VIN
RUN/SS
3.3k
TG1
220pF
LTC1709-8
ITH
SGND
PGOOD
5 VID BITS
VID0–VID4
EAIN
ATTENOUT
0.47µF
VIN
5V TO 28V
0.002Ω
1µH
SW1
S
BG1
PGND
SENSE1 +
SENSE1 –
TG2
BOOST2
BG2
VDIFFOUT
INTVCC
–
SENSE2 +
VOS +
SENSE2 –
0.002Ω
0.47µF
VOUT
1.3V TO 3.5V
40A
1µH
SW2
ATTENIN
VOS
S
BOOST1
+
0.1µF
10µF
35V
×4
10µF
+
COUT
1000µF
4V
×2
17097 F01
Figure 1. High Current 2-Phase Step-Down Converter
1
LTC1709-8/LTC1709-9
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
Input Supply Voltage (VIN).........................36V to – 0.3V
Topside Driver Voltages (BOOST1,2) .........42V to – 0.3V
Switch Voltage (SW1, 2) ..............................36V to – 5V
SENSE1+, SENSE2 +, SENSE1–,
SENSE2 – Voltages ........................ (1.1)INTVCC to – 0.3V
EAIN, VOS+, VOS–, EXTVCC, INTVCC, RUN/SS,
VBIAS, ATTENIN, ATTENOUT, PGOOD,
VID0–VID4, Voltages ...................................7V to – 0.3V
Boosted Driver Voltage (BOOST-SW) ..........7V to – 0.3V
PLLFLTR, PLLIN, VDIFFOUT Voltages .... INTVCC to – 0.3V
ITH Voltage ................................................2.7V to – 0.3V
Peak Output Current <1µs(TGL1,2, BG1,2) ................ 3A
INTVCC RMS Output Current ................................ 50mA
Operating Ambient Temperature Range
(Note 2) ................................................ – 40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
RUNN/SS
1
36 NC
SENSE1 +
2
35 TG1
SENSE1 –
3
34 SW1
EAIN
4
33 BOOST1
PLLFLTR
5
32 VIN
PLLIN
6
31 BG1
NC
7
30 EXTVCC
ITH
8
29 INTVCC
SGND
9
28 PGND
VDIFFOUT 10
LTC1709EG-8
LTC1709EG-9
27 BG2
VOS – 11
26 BOOST2
VOS + 12
25 SW2
SENSE2 – 13
24 TG2
SENSE2 + 14
23 PGOOD
ATTENOUT 15
22 VBIAS
ATTENIN 16
21 VID4
VID0 17
20 VID3
VID1 18
19 VID2
G PACKAGE
36-LEAD PLASTIC SSOP
TJMAX = 125°C, θJA = 90°C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VBIAS = 5V, VRUN/SS = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
VEAIN
Regulated Feedback Voltage
ITH Voltage = 1.2V (Note 4)
●
0.792
0.800
0.808
VSENSEMAX
Maximum Current Sense Threshold
VSENSE – = 5V
●
62
75
88
mV
IINEAIN
Feedback Current
(Note 4)
–5
– 50
nA
VLOADREG
Output Voltage Load Regulation
(Note 4)
Measured in Servo Loop, ∆ITH Voltage: 1.2V to 0.7V
Measured in Servo Loop, ∆ITH Voltage: 1.2V to 2V
0.1
– 0.1
0.5
– 0.5
%
%
●
●
V
VREFLNREG
Reference Voltage Line Regulation
VIN = 3.6V to 30V (Note 4)
VOVL
Output Overvoltage Threshold
Measured at VEAIN
UVLO
Undervoltage Lockout
VIN Ramping Down
gm
Transconductance Amplifier gm
ITH = 1.2V, Sink/Source 5µA (Note 4)
3
mmho
gmOL
Transconductance Amplifier Gain
ITH = 1.2V, (gmxZL; No Ext Load) (Note 4)
1.5
V/mV
IQ
Input DC Supply Current
Normal Mode
Shutdown
(Note 5)
EXTVCC Tied to VOUT, VOUT = 5V
VRUN/SS = 0V
470
20
IRUN/SS
Soft-Start Charge Current
VRUN/SS = 1.9V
– 0.5
–1.2
VRUN/SS
RUN/SS Pin ON Arming
VRUN/SS Rising
1.0
1.5
2
●
0.002
0.02
%/V
0.84
0.86
0.88
V
3
3.5
4
V
40
µA
µA
µA
1.9
V
LTC1709-8/LTC1709-9
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VBIAS = 5V, VRUN/SS = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VRUN/SSLO
RUN/SS Pin Latchoff Arming
VRUN/SS Rising from 3V
MIN
ISCL
RUN/SS Discharge Current
Soft Short Condition VEAIN = 0.5V, VRUN/SS = 4.5V
ISDLHO
Shutdown Latch Disable Current
VEAIN = 0.5V
ISENSE
Total Sense Pins Source Current
Each Channel: VSENSE1 –, 2 – = VSENSE1+, 2 + = 0V
– 85
– 60
µA
DFMAX
Maximum Duty Factor
In Dropout
98
99.5
%
TG1, 2 tr
TG1, 2 tf
Top Gate Transition Time:
Rise Time
Fall Time
(Note 6)
CLOAD = 3300pF
CLOAD = 3300pF
30
40
90
90
ns
ns
BG1, 2 tr
BG1, 2 tf
Bottom Gate Transition Time:
Rise Time
Fall Time
(Note 6)
CLOAD = 3300pF
CLOAD = 3300pF
30
20
90
90
ns
ns
TG/BG t1D
Top Gate Off to Bottom Gate On Delay
Synchronous Switch-On Delay Time
CLOAD = 3300pF Each Driver (Note 6)
90
ns
BG/TG t2D
Bottom Gate Off to Top Gate On Delay
Top Switch-On Delay Time
CLOAD = 3300pF Each Driver (Note 6)
90
ns
tON(MIN)
Minimum On-Time
Tested with a Square Wave (Note 7)
180
ns
0.5
TYP
MAX
UNITS
4.1
4.5
V
2
4
µA
1.6
5
µA
Internal VCC Regulator
VINTVCC
Internal VCC Voltage
6V < VIN < 30V, VEXTVCC = 4V
4.8
VLDO INT
INTVCC Load Regulation
ICC = 0 to 20mA, VEXTVCC = 4V
0.2
1.0
%
VLDO EXT
EXTVCC Voltage Drop
ICC = 20mA, VEXTVCC = 5V
80
160
mV
VEXTVCC
EXTVCC Switchover Voltage
ICC = 20mA, EXTVCC Ramping Positive
VLDOHYS
EXTVCC Switchover Hysteresis
ICC = 20mA, EXTVCC Ramping Negative
●
4.5
5.0
5.2
V
4.7
V
0.2
V
VID Parameters
VBIAS
Operating Supply Voltage Range
2.7
RATTEN
Resistance Between ATTENIN
and ATTENOUT Pins
LTC1709-8
LTC1709-9
ATTENERR
Resistive Divider Error
LTC1709-8: VID4 = 0; LTC1709-9
LTC1709-8: VID4 = 1
RPULLUP
VID0–VID4 Pull-Up Resistance
(Note 8)
VIDTHLOW
VID0–VID4 Logic Threshold Low
VIDTHHIGH
VID0–VID4 Logic Threshold High
VIDLEAK
VID0–VID4 Leakage
5.5
20
10
●
●
– 0.25
–0.35
kΩ
kΩ
0.25
0.25
40
%
%
kΩ
0.4
V
1
µA
1.6
VBIAS < VID0–VID4 < 7V
V
V
0.1
Oscillator and Phase-Locked Loop
fNOM
Nominal Frequency
VPLLFLTR = 1.2V
190
220
250
kHz
fLOW
Lowest Frequency
VPLLFLTR = 0V
120
140
160
kHz
fHIGH
Highest Frequency
VPLLFLTR ≥ 2.4V
280
320
360
kHz
RPLLIN
PLLIN Input Resistance
IPLLFLTR
Phase Detector Output Current
Sinking Capability
Sourcing Capability
RRELPHS
Controller 2-Controller 1 Phase
fPLLIN < fOSC
fPLLIN > fOSC
50
kΩ
– 15
15
µA
µA
180
Deg
3
LTC1709-8/LTC1709-9
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VRUN/SS = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.1
0.3
V
±1
µA
PGOOD Output
VPGL
PGOOD Voltage Low
IPGOOD = 2mA
IPGOOD
PGOOD Leakage Current
VPGOOD = 5V
VPG
PGOOD Trip Level, Either Controller
VEAIN with Respect to Set Output Voltage
VEAIN Ramping Negative
VEAIN Ramping Positive
–6
6
– 7.5
7.5
– 9.5
9.5
%
%
0.995
1
1.005
V/V
Differential Amplifier/Op Amp Gain Block
ADA
Gain
CMRRDA
Common Mode Rejection Ratio
0V < VCM < 5V
RIN
Input Resistance
Measured at VOS + Input
46
Note 1: Absolute Maximum Ratings are those values beyond which the
life of a device may be impaired.
Note 2: The LTC1709EG is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the – 40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
LTC1709EG: TJ = TA + (PD • 85°C/W)
Note 4: The LTC1709-8/LTC1709-9 are tested in a feedback loop that
servos VITH to a specified voltage and measures the resultant VEAIN.
55
dB
80
kΩ
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 6: Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.
Note 7: The minimum on-time condition corresponds to the on inductor
peak-to-peak ripple current ≥ 40% IMAX (see Minimum On-Time
Considerations in the Applications Information section).
Note 8: Each built-in pull-up resistor attached to the VID inputs also has a
series diode to allow input voltages higher than the VIDVCC supply without
damage or clamping (see the Applications Information section).
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TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Output Current
(Figure 12)
Efficiency vs Output Current
(Figure 12)
Efficiency vs Output Current
(Figure 12)
100
100
100
VOUT = 3.3V
VEXTVCC = 5V
IOUT = 20A
VEXTVCC = 5V
80
80
VIN = 8V
60
VIN = 12V
VIN = 20V
40
20
VOUT = 2V
VEXTVCC = 0V
FREQ = 200kHz
0
0.1
1
10
OUTPUT CURRENT (A)
100
170989 G01
4
VEXTVCC = 0V
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 5V
60
40
20
0
0.1
90
80
VIN = 12V
VOUT = 2V
FREQ = 200kHz
70
1
10
OUTPUT CURRENT (A)
100
5
10
15
20
VIN (V)
170989 G02
170989 G03
LTC1709-8/LTC1709-9
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TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Input Voltage
and Mode (Figure 12)
1000
5.05
INTVCC AND EXTVCC SWITCH VOLTAGE (V)
250
ON
EXTVCC VOLTAGE DROP (mV)
800
SUPPLY CURRENT (µA)
INTVCC and EXTVCC Switch
Voltage vs Temperature
EXTVCC Voltage Drop
600
400
200
200
150
100
50
SHUTDOWN
0
0
5
20
15
10
25
INPUT VOLTAGE (V)
30
0
35
0
10
30
20
CURRENT (mA)
40
170989 G04
50
4.95
4.90
4.85
4.80
4.70
– 50 – 25
100
125
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
Maximum Current Sense Threshold
vs Duty Factor
75
80
ILOAD = 1mA
70
60
4.8
4.7
50
VSENSE (mV)
4.9
VSENSE (mV)
INTVCC VOLTAGE (V)
50
25
75
0
TEMPERATURE (°C)
170989 G06
5.0
25
4.6
50
40
30
20
4.5
10
0
4.4
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
0
35
20
40
60
DUTY FACTOR (%)
80
Maximum Current Sense Threshold
vs VRUN/SS (Soft-Start)
80
0
100
50
100
0
25
75
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
170989 G08
170989 G07
170989 G09
Current Sense Threshold
vs ITH Voltage
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
90
80
VSENSE(CM) = 1.6V
80
70
76
40
60
VSENSE (mV)
VSENSE (mV)
60
VSENSE (mV)
EXTVCC SWITCHOVER THRESHOLD
4.75
170989 G05
Internal 5V LDO Line Regulation
5.1
INTVCC VOLTAGE
5.00
72
68
50
40
30
20
10
20
0
64
–10
–20
0
0
1
2
3
4
5
6
VRUN/SS (V)
170989 G10
60
0
1
3
4
2
COMMON MODE VOLTAGE (V)
5
170989 G11
–30
0
0.5
1
1.5
VITH (V)
2
2.5
170989 G12
5
LTC1709-8/LTC1709-9
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TYPICAL PERFOR A CE CHARACTERISTICS
Load Regulation
2.5
FCB = 0V
VIN = 15V
FIGURE 1
100
VOSENSE = 0.7V
2.0
–0.2
50
ISENSE (µA)
–0.1
VITH (V)
NORMALIZED VOUT (%)
SENSE Pins Total Source Current
VITH vs VRUN/SS
0.0
1.5
1.0
–0.3
0
–50
0.5
–0.4
0
1
3
2
LOAD CURRENT (A)
0
4
5
0
1
2
3
5
4
6
VRUN/SS (V)
–100
2
0
4
1629 G14
1629 G13
Maximum Current Sense
Threshold vs Temperature
1629 G15
RUN/SS Current vs Temperature
1.8
80
1.6
RUN/SS CURRENT (µA)
VSENSE (mV)
78
76
74
72
1.4
1.2
1.0
0.8
0.6
0.4
0.2
70
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
0
–50
–25
0
25
50
75
TEMPERATURE (°C)
170989 G16
100
125
170989 G17
Soft-Start (Figure 12)
Load Step (Figure 12)
VITH
1V/DIV
VOUT
50mV/DIV
VOUT
2V/DIV
VRUN/SS
2V/DIV
IOUT
0/20A
100ms/DIV
6
170989 G18
6
VSENSE COMMON MODE VOLTAGE (V)
20µs/DIV
170989 G19
LTC1709-8/LTC1709-9
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TYPICAL PERFOR A CE CHARACTERISTICS
Current Sense Pin Input Current
vs Temperature
10
VOUT = 5V
350
VFREQSET = 5V
33
31
29
27
50
25
0
75
TEMPERATURE (°C)
100
125
300
8
FREQUENCY (kHz)
EXTVCC SWITCH RESISTANCE (Ω)
CURRENT SENSE INPUT CURRENT (µA)
35
25
–50 –25
Oscillator Frequency
vs Temperature
EXTVCC Switch Resistance
vs Temperature
6
4
VFREQSET = 0V
150
100
50
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
0
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
170989 G22
170989 G21
Undervoltage Lockout
vs Temperature
Shutdown Latch Thresholds
vs Temperature
4.5
SHUTDOWN LATCH THRESHOLDS (V)
3.50
UNDERVOLTAGE LOCKOUT (V)
VFREQSET = OPEN
200
2
170989 G20
3.45
3.40
3.35
3.30
3.25
3.20
–50 –25
250
50
25
75
0
TEMPERATURE (°C)
100
125
170989 G23
LATCH ARMING
4.0
3.5
3.0
LATCHOFF
THRESHOLD
2.5
2.0
1.5
1.0
0.5
0
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
170989 G24
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PI FU CTIO S
RUN/SS (Pin 1): Combination of Soft-Start, Run Control
Input and Short-Circuit Detection Timer. A capacitor to
ground at this pin sets the ramp time to full current output.
Forcing this pin below 0.8V causes the IC to shut down all
internal circuitry. All functions are disabled in shutdown.
SENSE1+, SENSE2+ (Pins 2,14): The (+) Input to Each
Differential Current Comparator. The ITH pin voltage and
built-in offsets between SENSE– and SENSE+ pins in
conjunction with RSENSE set the current trip threshold.
SENSE1–, SENSE2– (Pins 3, 13): The (–) Input to the
Differential Current Comparators.
EAIN (Pin 4): Input to the error amplifier that compares the
feedback voltage to the internal 0.8V reference voltage.
This pin is normally connected to a resistive divider from
the output of the differential amplifier (DIFFOUT).
PLLFLTR (Pin 5): The phase-locked loop’s lowpass filter
is tied to this pin. Alternatively, this pin can be driven with
an AC or DC voltage source to vary the frequency of the
internal oscillator.
7
LTC1709-8/LTC1709-9
U
U
U
PI FU CTIO S
PLLIN (Pin 6): External Synchronization Input to Phase
Detector. This pin is internally terminated to SGND with
50kΩ. The phase-locked loop will force the rising top gate
signal of controller 1 to be synchronized with the rising
edge of the PLLIN signal.
NC (Pins 7, 36): Do not connect.
ITH (Pin 8): Error Amplifier Output and Switching Regulator Compensation Point. Both current comparator’s thresholds increase with this control voltage. The normal voltage
range of this pin is from 0V to 2.4V
SGND (Pin 9): Signal Ground. This pin is common to both
controllers. Route separately to the PGND pin.
VDIFFOUT (Pin 10): Output of a Differential Amplifier. This
pin provides true remote output voltage sensing. VDIFFOUT
normally drives an external resistive divider that sets the
output voltage.
VOS–, VOS+ (Pins 11, 12): Inputs to an Operational Amplifier. Internal precision resistors capable of being electronically switched in or out can configure it as a differential
amplifier or an uncommitted op amp.
ATTENOUT (Pin 15): Voltage Feedback Signal Resistively
Divided According to the VID Programming Code.
ATTENIN (Pin 16): The Input to the VID Controlled Resistive Divider.
VID0–VID4 (Pins 17,18, 19, 20, 21): VID Control Logic
Input Pins.
VBIAS (Pin 22): Supply Pin for the VID Control Circuit.
PGOOD (Pin 23): Open-Drain Logic Output. PGOOD is
pulled to ground when the voltage on the EAIN pin is not
within ±7.5% of its set point.
8
TG2, TG1 (Pins 24, 35): High Current Gate Drives for Top
N-Channel MOSFETS. These are the outputs of floating
drivers with a voltage swing equal to INTVCC superimposed on the switch node voltage SW.
SW2, SW1 (Pins 25, 34): Switch Node Connections to
Inductors. Voltage swing at these pins is from a Schottky
diode (external) voltage drop below ground to VIN.
BOOST2, BOOST1 (Pins 26, 33): Bootstrapped Supplies
to the Topside Floating Drivers. External capacitors are
connected between the BOOST and SW pins, and Schottky
diodes are connected between the BOOST and INTVCC
pins.
BG2, BG1 (Pins 27, 31): High Current Gate Drives for
Bottom N-Channel MOSFETS. Voltage swing at these pins
is from ground to INTVCC.
PGND (Pin 28): Driver Power Ground. Connect to sources
of bottom N-channel MOSFETS and the (–) terminals of
CIN.
INTVCC (Pin 29): Output of the Internal 5V Linear Low
Dropout Regulator and the EXTVCC Switch. The driver and
control circuits are powered from this voltage source.
Decouple to power ground with a 1µF ceramic capacitor
placed directly adjacent to the IC and minimum of 4.7µF
additional tantalum or other low ESR capacitor.
EXTVCC (Pin 30): External Power Input to an Internal
Switch. This switch closes and supplies INTVCC, bypassing the internal low dropout regulator whenever EXTVCC is
higher than 4.7V. See EXTVCC Connection in the Applications Information section. Do not exceed 7V on this pin
and ensure VEXTVCC ≤ VIN.
VIN (Pin 32): Main Supply Pin. Should be closely decoupled to the IC’s signal ground pin.
LTC1709-8/LTC1709-9
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FU CTIO AL DIAGRA
U
U
PLLIN
INTVCC
PHASE DET
fIN
50k
DUPLICATE FOR SECOND
CONTROLLER CHANNEL
RLP PLLFLTR
CLP
OSCILLATOR
DROP
OUT
DET
CLK2
S
Q
R
Q
–
CB
TG
TOP
+
CIN
BOT
SW
FORCE BOT
SWITCH
LOGIC
INTVCC
BOT
PGOOD
DB
BOOST
CLK1
VIN
BG
PGND
0.86V
+
SHDN
EAIN
–
+
0.74V
I1
R
+
R
A1
+
L
+
30k SENSE
0.86V
4(VFB)
–
VOS
–
–
30k SENSE
R
SLOPE
COMP
R
45k
45k
VOUT
EAIN
0.80V
VIN
–
EA
+
VREF
VIN
OV
4.7V
+
–
SHDN
RST
4(VFB)
INTVCC
+
6V
0.86V
ITH
RUN
SOFT
START
CC
RC
RUN/SS
INTERNAL
SUPPLY
SGND
0.80V
+
–
5V
LDO
REG
1.2µA
5V
COUT
2.4V
DIFFOUT
EXTVCC
RSENSE
+
+
+
VOS
INTVCC
–
–
CSS
ATTENIN
20k (LTC1709-8)
10k (LTC1709-9)
5-BIT VID DECODER
ATTENOUT
TYPICAL ALL
VID PINS
40k
R1
VID0
VID1
VID2
VID3
VID4
VBIAS
170989 FBD
9
LTC1709-8/LTC1709-9
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OPERATIO (Refer to Functional Diagram)
Main Control Loop
Low Current Operation
The LTC1709 uses a constant frequency, current mode
step-down architecture with inherent current sharing.
During normal operation, the top MOSFET is turned on
each cycle when the oscillator sets the RS latch, and
turned off when the main current comparator, I1, resets
the RS latch. The peak inductor current at which I 1 resets
the RS latch is controlled by the voltage on the ITH pin,
which is the output of the error amplifier EA. The differential amplifier, A1, produces a signal equal to the differential voltage sensed across the output capacitor but
re-references it to the internal signal ground (SGND)
reference. The EAIN pin receives a portion of this voltage
feedback signal at the DIFFOUT as determined by VID
logic input pins (VID0 to VID4) and is compared to the
internal reference voltage by the EA. When the load
current increases, it causes a slight decrease in the EAIN
pin voltage relative to the 0.8V reference, which in turn
causes the ITH voltage to increase until the average
inductor current matches the new load current. After the
top MOSFET has turned off, the bottom MOSFET is turned
on for the rest of the period.
The LTC1709 operates in a continuous, PWM control
mode. The resulting operation at low output currents
optimizes transient response at the expense of substantial
negative inductor current during the latter part of the
period. The level of ripple current is determined by the
inductor value, input voltage, output voltage and frequency of operation.
The top MOSFET drivers are biased from floating bootstrap capacitor CB, which normally is recharged during
each off cycle through an external Schottky diode. When
VIN decreases to a voltage close to VOUT, however, the loop
may enter dropout and attempt to turn on the top MOSFET
continuously. A dropout detector detects this condition
and forces the top MOSFET to turn off for about 400ns
every 10th cycle to recharge the bootstrap capacitor, CB.
The main control loop is shut down by pulling Pin 1 (RUN/
SS) low. Releasing RUN/SS allows an internal 1.2µA
current source to charge soft-start capacitor CSS. When
CSS reaches 1.5V, the main control loop is enabled with
the ITH voltage clamped at approximately 30% of its
maximum value. As CSS continues to charge, ITH is
gradually released allowing normal operation to resume.
When the RUN/SS pin is low, all LTC1709 functions are
shut down. If VOUT has not reached 70% of its nominal
value when CSS has charged to 4.1V, an overcurrent
latchoff can be invoked as described in the Applications
Information section.
10
Frequency Synchronization
The phase-locked loop allows the internal oscillator to be
synchronized to an external source via the PLLIN pin. The
output of the phase detector at the PLLFLTR pin is also the
DC frequency control input of the oscillator that operates
over a 140kHz to 310kHz range corresponding to a DC
voltage input from 0V to 2.4V. When locked, the PLL aligns
the turn on of the top MOSFET to the rising edge of the
synchronizing signal. When PLLIN is left open, the PLLFLTR
pin goes low, forcing the oscillator to minimum frequency.
Input capacitance ESR requirements and efficiency losses
are substantially reduced because the peak current drawn
from the input capacitor is effectively divided by two and
power loss is proportional to the RMS current squared. A
two stage, single output voltage implementation can
reduce input path power loss by 75% and radically reduce
the required RMS current rating of the input capacitor(s).
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
of the IC circuitry is derived from INTVCC. When the
EXTVCC pin is left open, an internal 5V low dropout
regulator supplies INTVCC power. If the EXTVCC pin is
taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on connecting EXTVCC to INTVCC.
This allows the INTVCC power to be derived from a high
efficiency external source such as the output of the regulator itself or a secondary winding, as described in the
Applications Information section. An external Schottky
diode can be used to minimize the voltage drop from
EXTVCC to INTVCC in applications requiring greater than
the specified INTVCC current. Voltages up to 7V can be
applied to EXTVCC for additional gate drive capability.
LTC1709-8/LTC1709-9
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OPERATIO
(Refer to Functional Diagram)
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both VOUT + and VOUT – benefits regulation in high current applications and/or applications having electrical interconnection losses. The AMPMD pin
allows selection of internal, precision feedback resistors
for high common mode rejection differencing applications, or direct access to the actual amplifier inputs
without these internal feedback resistors for other applications. The AMPMD pin is grounded to connect the internal
precision resistors in a unity-gain differencing application,
or tied to the INTVCC pin to bypass the internal resistors
and make the amplifier inputs directly available. The
amplifier is a unity-gain stable, 2MHz gain bandwidth,
>120dB open-loop gain design. The amplifier has an
output slew rate of 5V/µs and is capable of driving capacitive loads with an output RMS current typically up to
35mA. The amplifier is not capable of sinking current and
therefore must be resistively loaded to do so.
Power Good (PGOOD)
The PGOOD pin is connected to the drain of an internal
MOSFET. The MOSFET turns on when the output voltage
is not within ±7.5% of its nominal output level as determined by the feedback divider. When the output is within
±7.5% of its nominal value, the MOSFET is turned off
within 10µs and the PGOOD pin should be pulled up by an
external resistor to a source of up to 7V.
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the controllers
have been given time, as determined by the capacitor on
the RUN/SS pin, to charge up the output capacitors and
provide full-load current, the RUN/SS capacitor is then
used as a short-circuit timeout circuit. If the output voltage
falls to less than 70% of its nominal output voltage the
RUN/SS capacitor begins discharging assuming that the
output is in a severe overcurrent and/or short-circuit
condition. If the condition lasts for a long enough period
as determined by the size of the RUN/SS capacitor, the
controller will be shut down until the RUN/SS pin voltage
is recycled. This built-in latchoff can be overidden by
providing a current >5µA at a compliance of 5V to the
RUN/SS pin. This current shortens the soft-start period
but also prevents net discharge of the RUN/SS capacitor
during a severe overcurrent and/or short-circuit condition. Foldback current limiting is activated when the output
voltage falls below 70% of its nominal level whether or not
the short-circuit latchoff circuit is enabled.
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The basic LTC1709 application circuit is shown in Figure␣ 1 on the first page. External component selection
begins with the selection of the inductor(s) based on
ripple current requirements and continues with the
RSENSE1, 2 resistor selection using the calculated peak
inductor current and/or maximum current limit. Next, the
power MOSFETs and D1 and D2 are selected. The operating frequency and the inductor are chosen based mainly
on the amount of ripple current. Finally, CIN is selected for
its ability to handle the input ripple current (that
PolyPhaseTM operation minimizes) and COUT is chosen
with low enough ESR to meet the output ripple voltage
and load step specifications (also minimized with
PolyPhase). Current mode architecture provides inherent
current sharing between output stages. The circuit shown
in Figure␣ 1 can be configured for operation up to an input
voltage of 28V (limited by the external MOSFETs).
RSENSE Selection For Output Current
RSENSE1, 2 are chosen based on the required peak output
current. The LTC1709 current comparator has a maximum threshold of 75mV/RSENSE and an input common
mode range of SGND to 1.1(INTVCC). The current comparator threshold sets the peak inductor current, yielding
a maximum average output current IMAX equal to the peak
value less half the peak-to-peak ripple current, ∆IL.
PolyPhase is a trademark of Linear Technology Corporation.
11
LTC1709-8/LTC1709-9
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APPLICATIO S I FOR ATIO
Allowing a margin for variations in the LTC1709 and
external component values yields:
RSENSE = 2(50mV/IMAX)
Operating Frequency
effect of inductor value on ripple current and low current
operation must also be considered. The PolyPhase approach reduces both input and output ripple currents
while optimizing individual output stages to run at a lower
fundamental frequency, enhancing efficiency.
The LTC1709 uses a constant frequency, phase-lockable
architecture with the frequency determined by an internal
capacitor. This capacitor is charged by a fixed current
plus an additional current which is proportional to the
voltage applied to the PLLFLTR pin. Refer to PhaseLocked Loop and Frequency Synchronization for additional information.
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆IL per individual section, N,
decreases with higher inductance or frequency and
increases with higher VIN or VOUT:
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure␣ 2. As the operating frequency
is increased the gate charge losses will be higher, reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 310kHz.
where f is the individual output stage operating frequency.
PLLFLTR PIN VOLTAGE (V)
2.5
VOUT  VOUT 
 1−

fL 
VIN 
In a 2-phase converter, the net ripple current seen by the
output capacitor is much smaller than the individual
inductor ripple currents due to ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Application Note 77.
Figure 3 shows the net ripple current seen by the output
capacitors for the 1- and 2- phase configurations. The
output ripple current is plotted for a fixed output voltage as
the duty factor is varied between 10% and 90% on the
x-axis. The output ripple current is normalized against the
inductor ripple current at zero duty factor. The graph can
be used in place of tedious calculations, simplifying the
design process.
2.0
1.5
1.0
0.5
0
120
∆IL =
170
220
270
OPERATING FREQUENCY (kHz)
1.0
320
1709 F02
0.8
0.7
The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because
MOSFET gate charge and transition losses increase directly with frequency. In addition to this basic tradeoff, the
0.6
VO/fL
Inductor Value Calculation and Output Ripple Current
∆IO(P-P)
Figure 2. Operating Frequency vs VPLLFLTR
12
1-PHASE
2-PHASE
0.9
0.5
0.4
0.3
0.2
0.1
0
0.1
0.2
0.3 0.4 0.5 0.6 0.7
DUTY FACTOR (VOUT/VIN)
0.8
0.9
1709 F03
Figure 3. Normalized Output Ripple Current
vs Duty Factor [IRMS ≈ 0.3 (∆IO(P–P))]
LTC1709-8/LTC1709-9
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Accepting larger values of ∆IL allows the use of low
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is ∆IL
= 0.4(IOUT)/2, where IOUT is the total load current. Remember, the maximum ∆IL occurs at the maximum input
voltage. The individual inductor ripple currents are determined by the inductor, input and output voltages.
(see EXTVCC Pin Connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected
(VIN < 5V); then, sublogic-level threshold MOSFETs
(VGS(TH) < 1V) should be used. Pay close attention to the
BVDSS specification for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Inductor Core Selection
Selection criteria for the power MOSFETs include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS,
input voltage and maximum output current. When the
LTC1709 is operating in continuous mode the duty factors
for the top and bottom MOSFETs of each output stage are
given by:
Once the values for L1 and L2 are known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of more expensive
ferrite, molypermalloy, or Kool Mµ® cores. Actual core
loss is independent of core size for a fixed inductor value,
but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately,
increased inductance requires more turns of wire and
therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that inductance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive
than ferrite. A reasonable compromise from the same
manufacturer is Kool Mµ. Toroids are very space efficient, especially when you can use several layers of wire.
Because they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
controller with the LTC1709: one N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTVCC
voltage. This voltage is typically 5V during start-up
Main Switch Duty Cycle =
VOUT
VIN
V –V 
Synchronous Switch Duty Cycle =  IN OUT 


VIN
The MOSFET power dissipations at maximum output
current are given by:
2
I

V
PMAIN = OUT  MAX  1 + δ RDS(ON) +
VIN  2 

2 I
k VIN  MAX  CRSS f
 2 
( )
( )
(
)( )
2
I

V –V
PSYNC = IN OUT  MAX  1 + δ RDS(ON)
VIN
 2 
( )
where δ is the temperature dependency of RDS(ON) and k
is a constant inversely related to the gate drive current.
Both MOSFETs have I2R losses but the topside N-channel
equation includes an additional term for transition losses,
which peak at the highest input voltage. For VIN < 20V the
high current efficiency generally improves with larger
MOSFETs, while for VIN > 20V the transition losses rapidly
increase to the point that the use of a higher RDS(ON) device
with lower CRSS actual provides higher efficiency. The
Kool Mµ is a registered trademark of Magnetics, Inc.
13
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0.5
DC LOAD CURRENT
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized RDS(ON) vs temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs. CRSS is usually specified in the
MOSFET characteristics. The constant k = 1.7 can be
used to estimate the contributions of the two terms in the
main switch dissipation equation.
0.6
RMS INPUT RIPPLE CURRNET
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during a
short-circuit when the synchronous switch is on close to
100% of the period.
0.4
1-PHASE
2-PHASE
0.3
0.2
0.1
0
0.1
0.2
0.3 0.4 0.5 0.6 0.7
DUTY FACTOR (VOUT/VIN)
0.8
0.9
170989 F04
The Schottky diodes, D1 and D2 shown in Figure 1
conduct during the dead-time between the conduction of
the two large power MOSFETs. This helps prevent the
body diode of the bottom MOSFET from turning on,
storing charge during the dead-time, and requiring a
reverse recovery period which would reduce efficiency. A
1A to 3A Schottky (depending on output current) diode is
generally a good compromise for both regions of operation due to the relatively small average current. Larger
diodes result in additional transition losses due to their
larger junction capacitance.
CIN and COUT Selection
In continuous mode, the source current of each top
N-channel MOSFET is a square wave of duty cycle VOUT/
VIN. A low ESR input capacitor sized for the maximum
RMS current must be used. The details of a closed form
equation can be found in Application Note 77. Figure 4
shows the input capacitor ripple current for a 2-phase
configuration with the output voltage fixed and input
voltage varied. The input ripple current is normalized
against the DC output current. The graph can be used in
place of tedious calculations. The minimum input ripple
current can be achieved when the input voltage is twice the
output voltage
In the graph of Figure 4, the 2-phase local maximum input
RMS capacitor currents are reached when:
VOUT 2k − 1
=
4
VIN
where k = 1, 2
14
Figure 4. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages
These worst-case conditions are commonly used for
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. Always
consult the capacitor manufacturer if there is any
question.
It is important to note that the efficiency loss is proportional to the input RMS current squared and therefore a
2-phase implementation results in 75% less power loss
when compared to a single phase design. Battery/input
protection fuse resistance (if used), PC board trace and
connector resistance losses are also reduced by the
reduction of the input ripple current in a 2-phase system.
The required amount of input capacitance is further
reduced by the factor, 2, due to the effective increase in
the frequency of the current pulses.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically once the ESR requirement has been met, the RMS current rating generally far
exceeds the IRIPPLE(P-P) requirements. The steady state
output ripple (∆VOUT) is determined by:

1 
∆VOUT ≈ ∆IRIPPLE  ESR +

16 fCOUT 

LTC1709-8/LTC1709-9
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Where f = operating frequency of each stage, COUT =
output capacitance and ∆IRIPPLE = combined inductor
ripple currents.
The output ripple varies with input voltage since ∆IL is a
function of input voltage. The output ripple will be less than
50mV at max VIN with ∆IL = 0.4IOUT(MAX)/2 assuming:
COUT required ESR < 4(RSENSE) and
COUT > 1/(16f)(RSENSE)
The emergence of very low ESR capacitors in small,
surface mount packages makes very physically small
implementations possible. The ability to externally compensate the switching regulator loop using the I TH
pin(OPTI-LOOP compensation) allows a much wider selection of output capacitor types. OPTI-LOOP compensation effectively removes constraints on output capacitor
ESR. The impedance characteristics of each capacitor
type are significantly different than an ideal capacitor and
therefore require accurate modeling or bench evaluation
during design.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance
through-hole capacitors. The OS-CON semiconductor
dielectric capacitor available from Sanyo and the Panasonic
SP surface mount types have the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON type capacitors is recommended to reduce
the inductance effects.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum electrolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer surface mount capacitors offer very low ESR also but have
much lower capacitive density per unit volume. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. Several excellent
choices are the AVX TPS, AVX TPSV or the KEMET T510
series of surface mount tantalums, available in case heights
ranging from 2mm to 4mm. Other capacitor types include
Sanyo OS-CON, Nichicon PL series and Sprague 595D
series. Consult the manufacturer for other specific recommendations. A combination of capacitors will often result
in maximizing performance and minimizing overall cost
and size.
INTVCC Regulator
An internal P-channel low dropout regulator produces 5V
at the INTVCC pin from the VIN supply pin. The INTVCC
regulator powers the drivers and internal circuitry of the
LTC1709. The INTVCC pin regulator can supply up to 50mA
peak and must be bypassed to power ground with a
minimum of 4.7µF tantalum or electrolytic capacitor. An
additional 1µF ceramic capacitor placed very close to the
IC is recommended due to the extremely high instantaneous currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maximum junction temperature rating for the LTC1709 to be
exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTVCC pin. When the voltage applied to the EXTVCC pin
is less than 4.7V, all of the INTVCC load current is supplied
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (IIN)(VIN – INTVCC) and
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC1709 VIN
current is limited to less than 24mA from a 24V supply:
TJ = 70°C + (24mA)(24V)(85°C/W) = 119°C
Use of the EXTVCC pin reduces the junction temperature
to:
TJ = 70°C + (24mA)(5V)(85°C/W) = 80.2°C
The input supply current should be measured while the
controller is operating in continuous mode at maximum
VIN and the power dissipation calculated in order to
prevent the maximum junction temperature from being
exceeded.
15
LTC1709-8/LTC1709-9
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EXTVCC Connection
The LTC1709 contains an internal P-channel MOSFET
switch connected between the EXTVCC and INTVCC pins.
When the voltage applied to EXTVCC rises above 4.7V, the
internal regulator is turned off and an internal switch
closes, connecting the EXTVCC pin to the INTVCC pin
thereby supplying internal and MOSFET gate driving power
to the IC. The switch remains closed as long as the voltage
applied to EXTVCC remains above 4.5V. This allows the
MOSFET driver and control power to be derived from the
output during normal operation (4.7V < VEXTVCC < 7V) and
from the internal regulator when the output is out of
regulation (start-up, short-circuit). Do not apply greater
than 7V to the EXTVCC pin and ensure that EXTVCC < VIN +
0.3V when using the application circuits shown. If an
external voltage source is applied to the EXTVCC pin when
the VIN supply is not present, a diode can be placed in
series with the LTC1709’s VIN pin and a Schottky diode
between the EXTVCC and the VIN pin, to prevent current
from backfeeding VIN.
Significant efficiency gains can be realized by powering
INTVCC from the output, since the VIN current resulting
from the driver and control currents will be scaled by the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTVCC pin directly to VOUT. However, for 3.3V and other lower voltage regulators, additional circuitry is required to derive INTVCC power from the
output.
OPTIONAL EXTVCC
CONNECTION
5V < VSEC < 7V
+
The following list summarizes the four possible connections for EXTVCC:
1. EXTVCC left open (or grounded). This will cause INTVCC
to be powered from the internal 5V regulator resulting in
a significant efficiency penalty at high input voltages.
2. EXTVCC connected directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCC connected to an external supply. If an external
supply is available in the 5V to 7V range, it may be used to
power EXTVCC providing it is compatible with the MOSFET
gate drive requirements.
4. EXTVCC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTVCC to an outputderived voltage which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
inductive boost winding as shown in Figure 5a or the
capacitive charge pump shown in Figure 5b. The charge
pump has the advantage of simple magnetics.
Topside MOSFET Driver Supply (CB,DB) (Refer to
Functional Diagram)
External bootstrap capacitors CB1 and CB2 connected to
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor CB in the
Functional Diagram is charged though diode DB from
+
+
VIN
CIN
VIN
CIN
VIN
VIN
1N4148
TG1
LTC1709
+
T1
BAT85
RSENSE
VOUT
SW1
COUT
BG1
VOUT
L1
+
+
BG1
BAT85
VN2222LL
EXTVCC
RSENSE
SW1
N-CH
LTC1709
1µF
EXTVCC
0.22µF
TG1
VSEC
N-CH
BAT85
COUT
N-CH
N-CH
PGND
PGND
1709 F05b
1709 F05a
Figure 5a. Secondary Output Loop with EXTVCC Connection
16
Figure 5b. Capacitive Charge Pump for EXTVCC
LTC1709-8/LTC1709-9
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INTVCC when the SW pin is low. When the topside MOSFET
turns on, the driver places the CB voltage across the gatesource of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to VIN and the BOOST pin rises to VIN + VINTVCC.
The value of the boost capacitor CB needs to be 30 to 100
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of DB must be greater
than VIN(MAX).
The final arbiter when defining the best gate drive amplitude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
Output Voltage
The LTC1709 has a true remote voltage sense capablity.
The sensing connections should be returned from the load
back to the differential amplifier’s inputs through a common, tightly coupled pair of PC traces. The differential
amplifier corrects for DC drops in both the power and
ground paths. The differential amplifier output signal is
divided down and compared with the internal precision
0.8V voltage reference by the error amplifier.
Output Voltage Programming
Table 1. VID Output Voltage Programming
VID4
VID3
VID2
VID1
VID0
LTC1709-8
VRM8.4
LTC1709-9
VRM9.0
0
0
0
0
0
2.05V
1.850V
0
0
0
0
1
2.00V
1.825V
0
0
0
1
0
1.95V
1.800V
0
0
0
1
1
1.90V
1.775V
0
0
1
0
0
1.85V
1.750V
0
0
1
0
1
1.80V
1.725V
0
0
1
1
0
1.75V
1.700V
0
0
1
1
1
1.70V
1.675V
0
1
0
0
0
1.65V
1.650V
0
1
0
0
1
1.60V
1.625V
0
1
0
1
0
1.55V
1.600V
0
1
0
1
1
1.50V
1.575V
0
1
1
0
0
1.45V
1.550V
0
1
1
0
1
1.40V
1.525V
0
1
1
1
0
1.35V
1.500V
0
1
1
1
1
1.30V
1.475V
1
0
0
0
0
3.50V
1.450V
1
0
0
0
1
3.40V
1.425V
1
0
0
1
0
3.30V
1.400V
1
0
0
1
1
3.20V
1.375V
1
0
1
0
0
3.10V
1.350V
1
0
1
0
1
3.00V
1.325V
1
0
1
1
0
2.90V
1.300V
The output voltage is digitally programmed as defined in
Table 1 using the VID0 to VID4 logic input pins. The VID
logic inputs program a precision, 0.25% internal feedback
resistive divider. The LTC1709-8 has an output voltage
range of 1.30V to 3.5V in 50mV and 100mV steps. The
LTC1709-9 has an output voltage range of 1.10V to 1.85V
in 25mV steps.
1
0
1
1
1
2.80V
1.275V
1
1
0
0
0
2.70V
1.250V
1
1
0
0
1
2.60V
1.225V
1
1
0
1
0
2.50V
1.200V
1
1
0
1
1
2.40V
1.175V
1
1
1
0
0
2.30V
1.150V
1
1
1
0
1
2.20V
1.125V
Between the ATTENOUT pin and ground is a variable
resistor, R1, whose value is controlled by the five VID input
pins (VID0 to VID4). Another resistor, R2, between the
ATTENIN and the ATTENOUT pins completes the resistive
divider. The output voltage is thus set by the ratio of
(R1␣ +␣ R2) to R1.
1
1
1
1
0
2.10V
1.100V
1
1
1
1
1
No_CPU/
Shutdown*
No_CPU/
Shutdown*
*Represents codes without a defined output voltage as specified in Intel
specifications. The LTC1709 interprets these codes as a valid input and
produces an output voltage as follows:
LTC1709-8 (11111) = 2V
LTC1709-9 (11111) = 1.075V
17
LTC1709-8/LTC1709-9
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Each VID digital input is pulled up by a 40k resistor in
series with a diode from VBIAS. Therefore, it must be
grounded to get a digital low input, and can be either
floated or connected to VBIAS to get a digital high input. The
series diode is used to prevent the digital inputs from
being damaged or clamped if they are driven higher than
VBIAS. The digital inputs accept CMOS voltage levels.
VBIAS is the supply voltage for the VID section. It is
normally connected to INTVCC but can be driven from
other sources. If it is driven from another source, that
source must be in the range of 2.7V to 5.5V and must be
alive prior to enabling the LTC1709.
(
)
3V − 1.5V
CSS = 1.25s /µF CSS
1.2µA
By pulling the RUN/SS pin below 0.8V the LTC1709 is put
into low current shutdown (IQ < 40µA). The RUN/SS pins
can be driven directly from logic as shown in Figure 6.
Diode D1 in Figure 6 reduces the start delay but allows
CSS to ramp up slowly providing the soft-start function.
The RUN/SS pin has an internal 6V zener clamp (see
Functional Diagram).
3.3V OR 5V
The RUN/SS pin provides three functions: 1) Run/Shutdown, 2) soft-start and 3) a defeatable short-circuit latchoff
timer. Soft-start reduces the input power sources’ surge
currents by gradually increasing the controller’s current
limit ITH(MAX). The latchoff timer prevents very short,
extreme load transients from tripping the overcurrent
latch. A small pull-up current (>5µA) supplied to the RUN/
SS pin will prevent the overcurrent latch from operating.
The following explanation describes how the functions
operate.
An internal 1.2µA current source charges up the soft-start
capacitor, CSS. When the voltage on RUN/SS reaches
1.5V, the controller is permitted to start operating. As the
voltage on RUN/SS increases from 1.5V to 3.0V, the
internal current limit is increased from 25mV/RSENSE to
75mV/RSENSE. The output current limit ramps up slowly,
taking an additional 1.4s/µF to reach full current. The
output current thus ramps up slowly, reducing the starting
surge current required from the input power supply. If
RUN/SS has been pulled all the way to ground there is a
delay before starting of approximately:
18
tIRAMP =
INTVCC
VIN
Soft-Start/Run Function
tDELAY =
The time for the output current to ramp up is then:
(
)
1.5V
CSS = 1.25s /µF CSS
1.2µA
D1
RUN/SS
RSS*
RSS*
D1*
RUN/SS
CSS
CSS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
170989 F06
Figure 6. RUN/SS Pin Interfacing
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to latch off the
controllers when an overcurrent condition is detected. The
RUN/SS capacitor, CSS, is used initially to limit the inrush
current of both controllers. After the controllers have been
started and been given adequate time to charge up the
output capacitors and provide full load current, the RUN/
SS capacitor is used for a short-circuit timer. If the output
voltage falls to less than 70% of its nominal value after CSS
reaches 4.1V, CSS begins discharging on the assumption
that the output is in an overcurrent condition. If the
condition lasts for a long enough period as determined by
the size of the CSS, the controller will be shut down until the
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
tLO1 ≈ (CSS • 0.6V)/(1.2µA) = 5 • 105 (CSS)
LTC1709-8/LTC1709-9
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If the overload occurs after start-up, the voltage on CSS will
continue charging and will provide additional time before
latching off:
tLO2 ≈ (CSS • 3V)/(1.2µA) = 2.5 • 106 (CSS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, RSS, to the RUN/SS pin as
shown in Figure 6. This resistance shortens the soft-start
period and prevents the discharge of the RUN/SS capacitor during a severe overcurrent and/or short-circuit condition. When deriving the 5µA current from VIN as in the
figure, current latchoff is always defeated. The diode
connecting this pull-up resistor to INTVCC, as in Figure␣ 6,
eliminates any extra supply current during shutdown
while eliminating the INTVCC loading from preventing
controller start-up.
Why should you defeat current latchoff? During the
prototyping stage of a design, there may be a problem with
noise pickup or poor layout causing the protection circuit
to latch off the controller. Defeating this feature allows
troubleshooting of the circuit and PC layout. The internal
short-circuit and foldback current limiting still remains
active, thereby protecting the power supply system from
failure. A decision can be made after the design is complete whether to rely solely on foldback current limiting or
to enable the latchoff feature by removing the pull-up
resistor.
The value of the soft-start capacitor CSS may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capacitance is given by:
of 1.2V corresponds to a frequency of approximately
220kHz. The nominal operating frequency range of the
LTC1709 is 140kHz to 310kHz.
The phase detector used is an edge sensitive digital type
which provides zero degrees phase shift between the
external and internal oscillators. This type of phase detector will not lock up on input frequencies close to the
harmonics of the VCO center frequency. The PLL hold-in
range, ∆fH, is equal to the capture range, ∆fC:
∆fH = ∆fC = ±0.5 fO (150kHz-300kHz)
The output of the phase detector is a complementary pair
of current sources charging or discharging the external
filter network on the PLLFLTR pin. A simplified block
diagram is shown in Figure 7.
If the external frequency (fPLLIN) is greater than the oscillator frequency f0SC, current is sourced continuously,
pulling up the PLLFLTR pin. When the external frequency
is less than f0SC, current is sunk continuously, pulling
down the PLLFLTR pin. If the external and internal frequencies are the same but exhibit a phase difference, the
current sources turn on for an amount of time corresponding to the phase difference. Thus the voltage on the
PLLFLTR pin is adjusted until the phase and frequency of
the external and internal oscillators are identical. At this
stable operating point the phase comparator output is
open and the filter capacitor CLP holds the voltage. The
LTC1709 PLLIN pin must be driven from a low impedance
source such as a logic gate located close to the pin.
2.4V
CSS > (COUT )(VOUT)(10-4)(RSENSE)
The minimum recommended soft-start capacitor of CSS =
0.1µF will be sufficient for most applications.
PHASE
DETECTOR
CLP
EXTERNAL
OSC
PLLFLTR
PLLIN
Phase-Locked Loop and Frequency Synchronization
The LTC1709 has a phase-locked loop comprised of an
internal voltage controlled oscillator and phase detector.
This allows the top MOSFET turn-on to be locked to the
rising edge of an external source. The frequency range of
the voltage controlled oscillator is ±50% around the
center frequency fO. A voltage applied to the PLLFLTR pin
RLP
10k
50k
DIGITAL
PHASE/
FREQUENCY
DETECTOR
OSC
1709 F07
Figure 7. Phase-Locked Loop Block Diagram
19
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The loop filter components (CLP, RLP) smooth out the
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components CLP and RLP determine how fast the loop
acquires lock. Typically RLP =10k and CLP is 0.01µF to
0.1µF.
Minimum On-Time Considerations
Minimum on-time, tON(MIN), is the smallest time duration
that the LTC1709 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty cycle
applications may approach this minimum on-time limit
and care should be taken to ensure that:
tON(MIN) <
VOUT
()
VIN f
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1709 will begin to skip
cycles resulting in variable frequency operation. The output voltage will continue to be regulated, but the ripple
current and ripple voltage will increase.
The minimum on-time for the LTC1709 is generally less
than 200ns. However, as the peak sense voltage decreases, the minimum on-time gradually increases. This is
of particular concern in forced continuous applications
with low ripple current at light loads. If the duty cycle drops
below the minimum on-time limit in this situation, a
significant amount of cycle skipping can occur with correspondingly larger ripple current and voltage ripple.
If an application can operate close to the minimum
on-time limit, an inductor must be chosen that has a low
enough inductance to provide sufficient ripple amplitude
to meet the minimum on-time requirement. As a general
rule, keep the inductor ripple current of each phase equal
to or greater than 15% of IOUT(MAX) at VIN(MAX).
Voltage Positioning
Voltage positioning can be used to minimize peak-to-peak
output voltage excursion under worst-case transient loading conditions. The open-loop DC gain of the control loop
20
is reduced depending upon the maximum load step specifications. Voltage positioning can easily be added to the
LTC1709 by loading the ITH pin with a resistive divider
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage of the error amplifier, or 1.2V
(see Figure 8).
The resistive load reduces the DC loop gain while maintaining the linear control range of the error amplifier. The
worst-case peak-to-peak output voltage deviation due to
transient loading can theoretically be reduced to half or
alternatively the amount of output capacitance can be
reduced for a particular application. A complete explanation is included in Design Solutions 10 or the LTC1736
data sheet. (See www.linear-tech.com)
INTVCC
RT2
ITH
RT1
RC
LTC1709
CC
1709 F08
Figure 8. Active Voltage Positioning Applied to the LTC1709
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1709 circuits: 1) I2R losses, 2) Topside
MOSFET transition losses, 3) INTVCC regulator current
and 4) LTC1709 VIN current (including loading on the
differential amplifier output).
LTC1709-8/LTC1709-9
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1) I2R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resistor,
and input and output capacitor ESR. In continuous mode
the average output current flows through L and RSENSE,
but is “chopped” between the topside MOSFET and the
synchronous MOSFET. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one
MOSFET can simply be summed with the resistances of L,
RSENSE and ESR to obtain I2R losses. For example, if each
RDS(ON) = 10mΩ, RL = 10mΩ, and RSENSE = 5mΩ, then the
total resistance is 25mΩ. This results in losses ranging
from 2% to 8% as the output current increases from 3A to
15A per output stage for a 5V output, or a 3% to 12% loss
per output stage for a 3.3V output. Efficiency varies as the
inverse square of VOUT for the same external components
and output power level. The combined effects of increasingly lower output voltages and higher currents required
by high performance digital systems is not doubling but
quadrupling the importance of loss terms in the switching
regulator system!
2) Transition losses apply only to the topside MOSFET(s),
and are significant only when operating at high input
voltages (typically 12V or greater). Transition losses can
be estimated from:
Transition Loss = (1.7) VIN2 IO(MAX) CRSS f
3) INTVCC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from INTVCC to
ground. The resulting dQ/dt is a current out of INTVCC that
is typically much larger than the control circuit current. In
continuous mode, IGATECHG = (QT + QB), where QT and QB
are the gate charges of the topside and bottom side
MOSFETs.
Supplying INTVCC power through the EXTVCC switch input
from an output-derived source will scale the VIN current
required for the driver and control circuits by the ratio
(Duty Factor)/(Efficiency). For example, in a 20V to 5V
application, 10mA of INTVCC current results in approximately 3mA of VIN current. This reduces the mid-current
loss from 10% or more (if the driver was powered directly
from VIN) to only a few percent.
4) The VIN current has two components: the first is the
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control currents; the second is the current drawn from the differential
amplifier output. VIN current typically results in a small
(<0.1%) loss.
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% efficiency degradation in portable systems. It is very
important to include these “system” level losses in the
design of a system. The internal battery and input fuse
resistance losses can be minimized by making sure that
CIN has adequate charge storage and a very low ESR at
the switching frequency. A 50W supply will typically
require a minimum of 200µF to 300µF of capacitance
having a maximum of 10mΩ to 20mΩ of ESR. The
LTC1709 2-phase architecture typically halves this input
capacitance requirement over competing solutions. Other
losses including Schottky conduction losses during deadtime and inductor core losses generally account for less
than 2% total additional loss.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, VOUT shifts by an
amount equal to ∆ILOAD(ESR), where ESR is the effective
series resistance of COUT (∆ILOAD) also begins to charge or
discharge COUT generating the feedback error signal that
forces the regulator to adapt to the current change and
return VOUT to its steady-state value. During this recovery
time VOUT can be monitored for excessive overshoot or
ringing, which would indicate a stability problem. The
availability of the ITH pin not only allows optimization of
control loop behavior but also provides a DC coupled and
AC filtered closed loop response test point. The DC step,
rise time, and settling at this test point truly reflects the
closed loop response. Assuming a predominantly second
order system, phase margin and/or damping factor can be
21
LTC1709-8/LTC1709-9
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estimated using the percentage of overshoot seen at this
pin. The bandwidth can also be estimated by examining
the rise time at the pin. The ITH external components
shown in the Figure 1 circuit will provide an adequate
starting point for most applications.
The ITH series RC-CC filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
(from 0.2 to 5 times their suggested values) to optimize
transient response once the final PC layout is done and the
particular output capacitor type and value have been
determined. The output capacitors need to be decided
upon because the various types and values determine the
loop gain and phase. An output current pulse of 20% to
80% of full-load current having a rise time of <2µs will
produce output voltage and ITH pin waveforms that will
give a sense of the overall loop stability without breaking
the feedback loop. The initial output voltage step resulting
from the step change in output current may not be within
the bandwidth of the feedback loop, so this signal cannot
be used to determine phase margin. This is why it is
better to look at the Ith pin signal which is in the feedback
loop and is the filtered and compensated control loop
response. The gain of the loop will be increased by
increasing RC and the bandwidth of the loop will be
increased by decreasing CC. If RC is increased by the
same factor that CC is decreased, the zero frequency will
be kept the same, thereby keeping the phase the same in
the most critical frequency range of the feedback loop.
The output voltage settling behavior is related to the
stability of the closed-loop system and will demonstrate
the actual overall supply performance.
Automotive Considerations: Plugging into the
Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during operation. But before you connect, be advised: you are plugging
into the supply from hell. The main battery line in an
automobile is the source of a number of nasty potential
transients, including load-dump, reverse-battery, and
double-battery.
Load-dump is the result of a loose battery cable. When the
cable breaks connection, the field collapse in the alternator
can cause a positive spike as high as 60V which takes
several hundred milliseconds to decay. Reverse-battery is
just what it says, while double-battery is a consequence of
tow truck operators finding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 9 is the most straightforward approach to protect a DC/DC converter from the
ravages of an automotive power line. The series diode
prevents current from flowing during reverse-battery,
while the transient suppressor clamps the input voltage
during load-dump. Note that the transient suppressor
should not conduct during double-battery operation, but
must still clamp the input voltage below breakdown of the
converter. Although the LT1709 has a maximum input
voltage of 36V, most applications will be limited to 30V by
the MOSFET BVDSS.
50A IPK RATING
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
VIN
LTC1709
170989 F09
Figure 9. Automotive Application Protection
22
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Design Example
As a design example, assume VIN = 5V (nominal), VIN␣ =␣ 5.5V
(max), VOUT = 1.8V, IMAX = 20A, TA = 70°C and f␣ =␣ 300kHz.
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLFLTR pin
to the INTVCC pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
L≥
VOUT  VOUT 
 1−

VIN 
f ∆I 
( )
 1.8V 
≥
1−
 20A   5.5V 
300kHz 30% 

 2 
1.8V
(
)( )
≥ 1.35µH
A 1.5µH inductor will produce 27% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 11.4A. The minimum ontime occurs at maximum VIN:
tON(MIN) =
VOUT
1.8 V
=
= 1.1µs
VINf (5.5V )(300kHz)
The RSENSE resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
RSENSE =
PMAIN =
)]
)
The worst-case power disipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction temperature rise is:
2
PSYNC
5.5V − 1.8V  20A 
=

 1.48 0.013Ω
5.5V
 2 
( )(
)
= 1.29W
A short-circuit to ground will result in a folded back current
of about:
( )  = 6.6A

25mV
1  200ns 5.5V
ISC =
+
0.004Ω 2  1.5µH



The worst-case power disipated by the synchronous
MOSFET under short-circuit conditions at elevated ambient temperature and estimated 50°C junction temperature
rise is:
( ) (1.48)(0.013Ω)
5.5V − 1.8 V
6.6 A
5.5V
= 564mW
PSYNC =
50mV
≈ 0.004Ω
11.4A
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
RDS(ON) = 0.013Ω, CRSS = 300pF. At maximum input
voltage with TJ (estimated) = 110°C at an elevated ambient
temperature:
( ) [ ( )(
( )
(
(300kHz) = 0.65W
2
1.8V
10 1 + 0.005 110°C − 25°C
5.5V
2  20A 
0.013Ω + 1.7 5.5V 
 300pF
 2 
2
which is less than half of the normal, full-load dissipation.
Incidentally, since the load no longer dissipates power in
the shorted condition, total system power dissipation is
decreased by over 99%.
The duty factor for this application is:
DF =
VO 1.8 V
=
= 0.36
VIN
5V
23
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Using Figure 4, the RMS ripple current will be:
IINRMS = (20A)(0.23) = 4.6ARMS
An input capacitor(s) with a 4.6ARMS ripple current rating
is required.
The output capacitor ripple current is calculated by using
the inductor ripple already calculated for each inductor
and multiplying by the factor obtained from Figure␣ 3
along with the calculated duty factor. The output ripple in
continuous mode will be highest at the maximum input
voltage since the duty factor is < 50%. The maximum
output current ripple is:
( )
V
∆ICOUT = OUT 0.3 at 33% D F
fL
1.8 V
0.3
∆ICOUTMAX =
300kHz 1.5µH
(
= 1.2ARMS
)(
(
)
)
VOUTRIPPLE = 20mΩ 1.2ARMS = 24mVRMS
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1709. These items are also illustrated graphically in
the layout diagram of Figure␣ 10. Check the following in
your layout:
1) Are the signal and power grounds segregated? The
LTC1709 signal ground pin should return to the (–) plate
of COUT separately. The power ground returns to the
sources of the bottom N-channel MOSFETs, anodes of the
Schottky diodes, and (–) plates of CIN, which should have
as short lead lengths as possible.
2) Does the LTC1709 VOS+ pin connect to the point of
load? Does the LTC1709 VOS– pin connect to the load
return?
3) Are the SENSE – and SENSE + leads routed together with
minimum PC trace spacing? The filter capacitors between
SENSE + and SENSE – pin pairs should be as close as
possible to the LTC1709. Ensure accurate current sensing
with Kelvin connections at the current sense resistor.
24
4) Does the (+) plate of CIN connect to the drains of the
topside MOSFETs as closely as possible? This capacitor
provides the AC current to the MOSFETs. Keep the input
current path formed by the input capacitor, top and bottom
MOSFETs, and the Schottky diode on the same side of the
PC board in a tight loop to minimize conducted and
radiated EMI.
5) Is the INTVCC 1µF ceramic decoupling capacitor connected closely between INTVCC and the power ground pin?
This capacitor carries the MOSFET driver peak currents. A
small value is recommended to allow placement immediately adjacent to the IC.
6) Keep the switching nodes, SW1 (SW2), away from
sensitive small-signal nodes. Ideally the switch nodes
should be placed at the furthest point from the LTC1709.
7) Use a low impedance source such as a logic gate to drive
the PLLIN pin and keep the lead as short as possible.
The diagram in Figure 10 illustrates all branch currents in
a 2-phase switching regulator. It becomes very clear after
studying the current waveforms why it is critical to keep
the high-switching-current paths to a small physical size.
High electric and magnetic fields will radiate from these
“loops” just as radio stations transmit signals. The output
capacitor ground should return to the negative terminal of
the input capacitor and not share a common ground path
with any switched current paths. The left half of the circuit
gives rise to the “noise” generated by a switching regulator. The ground terminations of the sychronous MOSFETs
and Schottky diodes should return to the negative plate(s)
of the input capacitor(s) with a short isolated PC trace
since very high switched currents are present. A separate
isolated path from the negative plate(s) of the input
capacitor(s) should be used to tie in the IC power ground
pin (PGND) and the signal ground pin (SGND). This
technique keeps inherent signals generated by high current pulses from taking alternate current paths that have
finite impedances during the total period of the switching
regulator. External OPTI-LOOP compensation allows overcompensation for PC layouts which are not optimized but
this is not the recommended design procedure.
LTC1709-8/LTC1709-9
U
W
U
U
APPLICATIO S I FOR ATIO
SW1
L1
RSENSE1
D1
VIN
VOUT
RIN
CIN
+
+
SW2
BOLD LINES INDICATE
HIGH, SWITCHING
CURRENT LINES.
KEEP LINES TO A
MINIMUM LENGTH.
L2
COUT
RL
RSENSE2
D2
170989 F10
Figure 10. Instantaneous Current Path Flow in a Multiple Phase Switching Regulator
Simplified Visual Explanation of How a 2-Phase
Controller Reduces Both Input and Output RMS Ripple
Current
A multiphase power supply significantly reduces the
amount of ripple current in both the input and output
capacitors. The RMS input ripple current is divided by, and
the effective ripple frequency is multiplied up by the
number of phases used (assuming that the input voltage
is greater than the number of phases used times the output
voltage). The output ripple amplitude is also reduced by,
and the effective ripple frequency is increased by the
number of phases used. Figure 11 graphically illustrates
the principle.
SINGLE PHASE
SW V
ICIN
DUAL PHASE
SW1 V
SW2 V
IL1
ICOUT
IL2
ICIN
ICOUT
RIPPLE
1709 F11
Figure 11. Single and 2-Phase Current Waveforms
25
LTC1709-8/LTC1709-9
U
W
U
U
APPLICATIO S I FOR ATIO
The worst-case RMS ripple current for a single stage
design peaks at an input voltage of twice the output
voltage. The worst-case RMS ripple current for a two stage
design results in peak outputs of 1/4 and 3/4 of input
voltage. When the RMS current is calculated, higher
effective duty factor results and the peak current levels are
divided as long as the currents in each stage are balanced.
Refer to Application Note 19 for a detailed description of
how to calculate RMS current for the single stage switching regulator. Figures 3 and 4 help to illustrate how the
input and output currents are reduced by using an additional phase. The input current peaks drop in half and the
frequency is doubled for this 2-phase converter. The input
capacity requirement is thus reduced theoretically by a
factor of four! Ceramic input capacitors with their
unbeatably low ESR characteristics can be used.
Figure 4 illustrates the RMS input current drawn from the
input capacitance vs the duty cycle as determined by the
ratio of input and output voltage. The peak input RMS
current level of the single phase system is reduced by 50%
in a 2-phase solution due to the current splitting between
the two stages.
26
An interesting result of the 2-phase solution is that the VIN
which produces worst-case ripple current for the input
capacitor, VOUT = VIN/2, in the single phase design produces zero input current ripple in the 2-phase design.
The output ripple current is reduced significantly when
compared to the single phase solution using the same
inductance value because the VOUT/L discharge current
term from the stage that has its bottom MOSFET on
subtracts current from the (VIN - VOUT)/L charging current
resulting from the stage which has its top MOSFET on. The
output ripple current is:
∆IRIPPLE =
( ) 

2VOUT  1 − 2D 1 − D
fL  1 − 2D + 1



where D is duty factor.
The input and output ripple frequency is increased by the
number of stages used, reducing the output capacity
requirements. When VIN is approximately equal to 2(VOUT)
as illustrated in Figures 3 and 4, very low input and output
ripple currents result.
LTC1709-8/LTC1709-9
U
PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
G Package
36-Lead Plastic SSOP (0.209)
(LTC DWG # 05-08-1640)
12.67 – 12.93*
(0.499 – 0.509)
36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19
7.65 – 7.90
(0.301 – 0.311)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
5.20 – 5.38**
(0.205 – 0.212)
1.73 – 1.99
(0.068 – 0.078)
0° – 8°
0.13 – 0.22
(0.005 – 0.009)
0.55 – 0.95
(0.022 – 0.037)
NOTE: DIMENSIONS ARE IN MILLIMETERS
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.152mm (0.006") PER SIDE
**DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.254mm (0.010") PER SIDE
0.65
(0.0256)
BSC
0.25 – 0.38
(0.010 – 0.015)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
0.05 – 0.21
(0.002 – 0.008)
G36 SSOP 1098
27
LTC1709-8/LTC1709-9
U
TYPICAL APPLICATIO
LTC1709-8
0.1µF
1000pF
2.7k
RUNN/SS
NC
2
SENSE1 +
TG1
3
SENSE1 –
SW1
4
10k
INTVCC
6
51k
15k
5
7
6.8k
8
680pF
100pF
9
10
11
PLLIN
VIN
BG1
NC
EXTVCC
ITH
INTVCC
SGND
VDIFFOUT
PGND
BG2
BOOST2
VOS +
SW2
13
SENSE2 –
TG2
14
SENSE2 +
PGOOD
15
470pF
BOOST1
PLLFLTR
VOS –
12
1000pF
EAIN
16
17
18
ATTENOUT
VBIAS
ATTENIN
VID4
VID0
VID3
VID1
VID2
36
L1
35
0.004Ω
34
0.22µF
33
M1
M2
32
D1
MBRS140T3
31
30
10Ω
5V (OPT)
CIN
47µF 35V
+
29
28
+
1
0.1µF
10µF
COUT
VIN
5V TO 28V
27
26
25
M3
0.22µF
M4
VOUT
1.3V TO 3.5V
20A
0.004Ω
24
100k
23
L2
PGOOD
22
21
D2
MBRS140T3
0.1µF
10Ω
20
19
SWITCHING FREQUENCY = 200kHz
CIN: 5A RIPPLE CURRENT RATING REQUIRED
COUT: 4 × 180µF/4V PANASONIC SP
L1 TO L2: 1.5µH SUMIDA CEP125-1R5MC
M1 TO M4: FAIRCHILD FDS7760A
OR Siliconix Si4430
VID INPUTS
170989 F12
Figure 12. 1.3V to 3.5V/20A CPU Power Supply with Active Voltage Positioning
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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LTC1708-PG
Dual High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator
with 5-Bit VID and Power Good Indication
1.3V ≤ VOUT ≤ 3.5V, Current Mode Ensures
Accurate Current Sharing, 3.5V ≤ VIN ≤ 36V
LTC1709
High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator
with 5-Bit VID and Fault Coupling Control
1.3V ≤ VOUT ≤ 3.5V, Current Mode Ensures
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LTC1709-7
High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator
with 5-Bit Mobile VID, and Three Low-Current Modes
0.9V ≤ VOUT ≤ 2V, Current Mode Ensures
Accurate Current Sharing, 3.5V ≤ VIN ≤ 36V
LTC1735
High Efficiency Synchronous Step-Down Controller
Burst ModeTM Operation, 16-Pin Narrow SSOP,
Fault Protection, 3.5V ≤ VIN ≤ 36V
LTC1736
High Efficiency Synchronous Step-Down Controller with 5-Bit VID
Output Fault Protection, Power Good, GN-24,
3.5V ≤ VIN ≤ 36V, 0.925V ≤ VOUT ≤ 2V
Adaptive Power and Burst Mode are trademarks of Linear Technology Corporation.
28
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
170989f LT/LCG 0900 4K • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2000