LINER LTC1773EMS-TR Synchronous step-down dc/dc controller Datasheet

LTC1773
Synchronous Step-Down
DC/DC Controller
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FEATURES
DESCRIPTIO
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The LTC®1773 is a current mode synchronous buck regulator controller that drives external complementary power
MOSFETs using a fixed frequency architecture. The operating supply range is from 2.65V to 8.5V, making it
suitable for 1- or 2-cell lithium-ion battery powered applications. Burst Mode® operation provides high efficiency at
low load currents. 100% duty cycle provides low dropout
operation which extends operating time in battery-operated systems.
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High Efficiency: Up to 95%
Constant Frequency 550kHz Operation
VIN from 2.65V to 8.5V
VOUT from 0.8V to VIN
OPTI-LOOP® Compensation Minimizes COUT
Synchronizable up to 750kHz
Selectable Burst Mode Operation
µA
Low Quiescent Current: 80µ
Low Dropout Operation: 100% Duty Cycle
Secondary Winding Regulation
Soft-Start
Current Mode Operation for Excellent Line and
Load Transient Response
Low Shutdown IQ = 10µA
±1.5% Reference Accuracy
Precision 2.5V Undervoltage Lockout
Available in 10-Lead MSOP
The operating frequency is internally set at 550kHz, allowing the use of small surface mount inductors. For switching-noise sensitive applications, it can be synchronized up
to 750kHz. Peak current limit is user programmable with
an external high side sense resistor. A SYNC/FCB control
pin guarantees regulation of secondary windings regardless of load on the main output by forcing continuous
operation. Burst Mode operation is inhibited during synchronization or when the SYNC/FCB pin is pulled low to
reduce noise and RF interference. Soft-start is provided by
an external capacitor.
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APPLICATIO S
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Cellular Telephones
RF PA Supplies
Portable Instruments
Wireless MODEMS
Distributed Power Systems
Notebook and Palm Top Computers, PDAs
Single and Dual Cell Lithium-Ion Powered Devices
Synchronous rectification increases efficiency and eliminates the need for a Schottky diode, saving components
and board space. The LTC1773 comes in a 10-lead MSOP
package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
OPTI-LOOP and Burst Mode are registered trademarks of Linear Technology Corporation.
Protected by U.S. Patents, including 5481178, 6580258, 6304066, 6127815.
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TYPICAL APPLICATIO
VIN
2.65V TO 8.5V
High Efficiency
100
+
VIN
SYNC/FCB
90
ITH
47pF
220pF
L1
3µH
TG
LTC1773
30k
SW
VFB
GND
VOUT
2.5V
BG
Si9801DY
+ COUT
180µF
80.6k
VIN = 5V
85
VIN = 8V
80
75
70
65
60
L = SUMIDA CDRH6D28-3R0
55
169k
1773 F01
Figure 1. Step-Down Converter
EFFICIENCY (%)
RUN/SS SENSE–
0.1µF
VIN = 3.3V
95
CIN
68µF
RSENSE
0.025Ω
1
100
1000
10
OUTPUT CURRENT (mA)
5000
1773 F1b
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LTC1773
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
Input Supply Voltage .............................. –0.3V to 10.0V
ITH Voltage ................................................ –0.3V to 2.5V
RUN/SS, VFB, SENSE– Voltages .................. –0.3V to VIN
SYNC/FCB Voltage ...................................... –0.3V to VIN
BG, TG Voltages...........................................–0.3V to VIN
SW Voltage ...................................................– 5V to 11V
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
ITH
RUN/SS
SYNC/FCB
VFB
GND
1
2
3
4
5
10
9
8
7
6
SW
SENSE–
VIN
TG
BG
LTC1773EMS
MS PART MARKING
MS PACKAGE
10-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 120°C/W
LTMV
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, TA = 25°C. VIN = 5V unless otherwise specified.
SYMBOL
PARAMETER
IVFB
Feedback Current
(Note 4)
VFB
Regulated Feedback Voltage
(Note 4)
∆VOVL
∆Output Overvoltage Lockout
∆VOVL = VOVL – VFB
∆VFB
Reference Voltage Line Regulation
VIN = 2.7V to 8.5V (Note 4)
VLOADREG
Output Voltage Load Regulation
ITH at 1.0V (Note 4)
IS
CONDITIONS
MIN
TYP
20
60
nA
0.788
0.80
0.812
V
40
60
80
mV
0.002
0.02
%/V
0.2
0.8
%
ITH at 0.6V (Note 4)
–0.2
–0.8
%
Input DC Bias Current
Normal Mode
(Note 5)
VIN = 5V, VITH = OPEN, VSYNC/MODE = OPEN
400
600
Burst Mode Operation
VITH = 0V, VIN = 5V, VSYNC/MODE = OPEN
80
Shutdown
VRUN/SS = 0V, 2.7V < VIN < 8.5V
10
Shutdown
VRUN/SS = 0V, VIN < 2.4V
●
MAX
UNITS
µA
µA
30
µA
2
5
µA
0.4
0.7
1.0
V
VRUN/SS = 0V
0.75
1.5
2.5
µA
Auxiliary Feedback Threshold
VSYNC/FCB Ramping Negative
0.76
0.8
0.84
V
SYNC/FCB Pull-Up Current
VSYNC/FCB = 0V
0.1
0.4
1.0
µA
fOSC
Oscillator Frequency
VFB = 0.8V
500
550
600
kHz
VUVLO
Undervoltage Lockout
2.35
2.5
2.65
V
2.65
2.8
V
VRUN/SS
RUN/SS Threshold
IRUN/SS
Soft-Start Current Source
VSYNC/FCB
ISYNC/FCB
VFB = 0V
∆VSENSE(MAX)
55
VIN Ramping Down from 3V
●
VIN Ramping Up from 0V
●
Maximum Current Sense Voltage
●
85
kHz
100
115
mV
TG tr
Top Gate Drive Rise Time
CLOAD = 3000pF (Note 6)
45
160
ns
TG tf
Top Gate Drive Fall Time
CLOAD = 3000pF (Note 6)
48
150
ns
BG tr
Bottom Gate Drive Rise Time
CLOAD = 3000pF (Note 6)
80
180
ns
BG tf
Bottom Gate Drive Fall Time
CLOAD = 3000pF (Note 6)
45
150
ns
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LTC1773
ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC1773 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:
LTC1773: TJ = TA + (PD • 120°C/W)
Note 4: The LTC1773 is tested in a feedback loop which servos VFB to the
balance point for the error amplifier (VITH = 0.8V)
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 6: Rise and fall times are measured using 10% and 90% levels.
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TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
Efficiency vs Load Current
VIN = 5V
100 VOUT = 2.5V
SEE FIGURE 1
VIN = 3.3V
Burst Mode
OPERATION
70
SYNC(750kHz)
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
95
90
80
VIN = 5V
80
VIN = 8V
70
FORCED CONTINUOUS
50
10
100
1000
OUTPUT CURRENT (mA)
1
75
50
1
5000
10
100
1000
OUTPUT CURRENT (mA)
2
10,000
500
RSENSE = 0.025Ω
350 VOUT = 1.8V
Si9801DY
RSENSE = 0.025Ω
SEE FIGURE 1
– 0.05
450
INPUT CURRENT (µA)
VIN-VOUT (mV)
250
200
150
100
– 0.25
2.5
1.0 1.5 2.0
LOAD CURRENT (A)
3.0
3.5
1773 G16
SYNC TO 750kHz
400
300
– 0.20
10
Input and Shutdown Currents
vs Input Voltage
400
0
– 0.15
6
8
INPUT VOLTAGE (V)
1773 G03
VIN-VOUT Dropout Voltage
vs Load Current
– 0.10
4
1773 G02
Load Regulation
0.5
85
IOUT = 100mA
1773 G01
0
IOUT = 1A
90
80
60
60
– 0.30
VOUT = 2.5V
SEE FIGURE 1
VOUT = 2.5V
SEE FIGURE 1
100
90
NORMALIZED VOUT (%)
Efficiency vs Input Voltage
100
350
VOUT = 1.8V
Si9801DY
RSENSE = 0.025Ω
L = CDRH6D28-3RO
300
250
200
Burst Mode OPERATION
150
100
50
50
0
0
0
500
1500
2000
1000
LOAD CURRENT (mA)
2500
1773 G05
SHUTDOWN
2
4
8
6
INPUT VOLTAGE (V)
10
1773 G06
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LTC1773
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TYPICAL PERFOR A CE CHARACTERISTICS
120
100
80
60
40
20
0
0.5
1.0
1.5
2.0 2.5 3.0
VRUN/SS (V)
3.5
4.0
105
100
95
85
– 60 – 40 – 20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
1773 G07
100
80
60
Burst Mode
OPERATION
40
20
0
560
2.5
550
2.0
540
530
520
– 60 –40 –20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
FORCED
CONTINUOUS
0
0.2
0.4
0.6 0.8 1.0
VITH (V)
1.2
1.4
1.6
1773 G09
RUN/SS Pin Current vs
Temperature
RUN/SS CURRENT (µA)
FREQUENCY (kHz)
120
1773 G08
Oscillator Frequency vs
Temperature
Burst Mode Operation
ILOAD = 100mA
SEE FIGURE 9
VOUT
20mV/DIV
1.5
IL
500mA/DIV
1.0
0.5
– 60 –40 –20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
VIN = 5V
VOUT = 2.5V
20µs/DIV
1773 G12
1773 G11
1773 G10
Load Step (Burst Mode
Operation)
Start-Up
VOUT
2V/DIV
Maximum Current Sense
Threshold vs VITH
MAXIMUM CURRENT SENSE THRESHOLD (mV)
Maximum Current Sense
Threshold vs Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
Maximum Current Sense
Threshold vs VRUN/SS
Load Step (Continuous
Mode)
SEE FIGURE 9
SEE FIGURE 9
SEE FIGURE 9
VOUT
100mV/DIV
VOUT
100mV/DIV
IL
2A/DIV
IL
2A/DIV
VRUN/SS
1V/DIV
IL
2A/DIV
VIN = 5V
VOUT = 2.5V
40ms/DIV
1773 G13
VIN = 5V
100µs/DIV
VOUT = 2.5V
100mA TO 5A LOAD STEP
1773 G14
100µs/DIV
VIN = 5V
VOUT = 2.5V
100mA TO 5A LOAD STEP
1773 G15
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LTC1773
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PIN FUNCTIONS
ITH (Pin 1): Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 1.2V.
Under high duty cycle and nearing current limit, ITH can
swing up to 2.4V.
RUN/SS (Pin 2): Combination of Soft-Start and Run
Control Inputs. A capacitor to ground at this pin sets the
ramp time to full current output. The time is approximately
0.8s/µF. Forcing this pin below 0.4V shuts down all the
circuitry.
SYNC/FCB (Pin 3): Multifunction Pin. This pin performs
three functions: 1) secondary winding feedback input, 2)
external clock synchronization and 3) Burst Mode operation or forced continuous mode select. For secondary
winding applications, connect to a resistive divider from
the secondary output. To synchronize with an external
clock, apply a TTL/CMOS compatible clock with a frequency between 585kHz and 750kHz. To select Burst
Mode operation, tie SYNC/FCB to VIN. Grounding this pin
forces continuous operation.
VFB (Pin 4): Feedback Pin. Receives the feedback voltage
from an external resistive divider across the output. Do not
use more than 0.01µF of feedforward capacitance from FB
to the output.
GND (Pin 5): Ground Pin.
BG (Pin 6): Bottom Gate Driver of External N-Channel
Power MOSFET. This pin swings from 0V to VIN.
TG (Pin 7): Top Gate Driver of External P-Channel Power
MOSFET. This pin swings from 0V to VIN.
VIN (Pin 8) : Main Supply Pin. Must be closely decoupled
to GND (pin 5).
SENSE–(Pin 9): The Negative Input to the Current Comparator. A sense resistor between this pin and VIN sets the
peak current in the top switch. Connect this pin to the
source of the external P-Channel power MOSFET.
SW (Pin 10): Switch Node Connection to Inductor. This
pin connects to the drains of the external main and
synchronous power MOSFET switches.
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FUNCTIONAL DIAGRA
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BURST
DEFEAT
X
0.4µA
Y = “0” ONLY WHEN X IS A CONSTANT “1”
Y
SLOPE
COMP
SYNC/FCB 3
8 VIN
OSC
–
SYNC
DEFEAT
EN
+
0.8V
FREQ
SHIFT
1.5µA
+
UVLO
TRIP = 2.5V
ICOMP
BURST
COMP
ITH
1
RUN/
RUN/SOFT
SOFT-START
START
–
OVDET
0.86V
+
0.22V
–
0.8V REF
2
RUN/SS
SLEEP
50mV
EA
+
–
–
S
Q
R
Q
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
7 TG
ANTI
SHOOT-THRU
+
6 BG
SHUTDOWN
5 GND
IRCMP
–
FCB
+
10 SW
–
0.8V
+
4
+
VFB
9 SENSE –
–
0.6V
0.4V
Figure 2.
1773 FD
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LTC1773
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OPERATIO
(Refer to Functional Diagram)
Main Control Loop
The LTC1773 uses a constant frequency, current mode
step- down architecture to drive an external pair of complementary power MOSFETs. During normal operation, the
external top P-channel power MOSFET turns on each cycle
when the oscillator sets the RS latch, and turns off when
the current comparator ICOMP resets the RS latch. The
peak inductor current at which ICOMP resets the RS latch
is controlled by the voltage on the ITH pin, which is the
output of error amplifier EA. The VFB pin, described in the
Pin Functions section, allows EA to receive an output
feedback voltage from an external resistive divider. When
the load current increases, it causes a slight decrease in
the feedback 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. While the
top P-channel MOSFET is off, the bottom N-channel
MOSFET is turned on until either the inductor current
starts to reverse, as indicated by the current reversal
comparator IRCMP, or the beginning of the next cycle.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 1.5µA
current source to charge the external soft-start capacitor
CSS. When CSS reaches 0.7V, the main control loop is
enabled with the internal buffered ITH voltage clamped at
approximately 5% of its maximum value. As CSS continues to charge, the internal buffered ITH is gradually released allowing normal operation to resume.
An overvoltage comparator, 0V, guards against transient
overshoots (>7.5%) as well as other more serious conditions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned on
until the overvoltage condition is cleared.
Burst Mode Operation
The LTC1773 is capable of Burst Mode operation in which
the external power MOSFETs operate intermittently based
on load demand. To enable Burst Mode operation, simply
allow the SYNC/FCB pin to float or connect it to a logic
high. To disable Burst Mode operation and force continuous mode, connect the SYNC/FCB pin to GND. The threshold voltage between Burst Mode operation and forced
continuous mode is 0.8V. This can be used to assist in
secondary winding regulation as described in Auxiliary
Winding Control Using SYNC/FCB Pin in the Applications
Information section.
When the converter operates in Burst Mode operation the
peak current of the inductor is set to approximately a third
of the maximum peak current value during normal operation even though the voltage at the ITH pin indicates a lower
value. The voltage at the ITH pin drops when the inductor’s
average current is greater than the load requirement. As
the ITH voltage drops below 0.22V, the BURST comparator trips, causing the internal sleep line to go high and turn
off both power MOSFETs.
The circuit enters sleep mode with both power MOSFETs
turned off. In sleep mode, the internal circuitry is partially
turned off, reducing the quiescent current to about 80µA.
The load current is now being supplied from the output
capacitor. When the output voltage drops, causing ITH to
rise above 0.27V, the internal sleep line goes low, and the
LTC1773 resumes normal operation. The next oscillator
cycle will turn on the external top MOSFET and the switching cycle repeats.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 55kHz, 1/10 the nominal
frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing
runaway. The oscillator’s frequency will gradually increase to 550kHz after VFB rises above 0.4V.
Frequency Synchronization
The LTC1773 can be synchronized with an external TTL/
CMOS compatible clock signal. The frequency range of
this signal must be from 585kHz to 750kHz. Do not
synchronize the LTC1773 below 585kHz as this may cause
abnormal operation and an undesired frequency spectrum. The top MOSFET turn-on follows the rising edge of
the external source.
When the LTC1773 is clocked by an external source, Burst
Mode operation is disabled; the LTC1773 then operates in
PWM pulse skipping mode preventing current reversal. In
this mode, when the output load is very low, current
comparator ICOMP remains tripped for more than one cycle
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LTC1773
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OPERATIO
(Refer to Functional Diagram)
and forces the main switch to stay off for the same number
of cycles. Increasing the output load current slightly,
above the minimum required for discontinuous conduction mode, allows constant frequency PWM.
Frequency synchronization is inhibited when the feedback
voltage, VFB, is below 0.6V. This prevents the external
clock from interfering with the frequency foldback for
short-circuit protection.
Low Supply Operation
The LTC1773 is designed to operate down to a 2.65V
supply voltage. For proper operation at this low input
voltage, sub-logic level MOSFETs are required. When the
value of the output voltage is very close to the input
voltage, the converter is running at high duty cycles or in
dropout where the main switch is on continuously. See
Efficiency Considerations in the Applications Information
section.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle until it reaches 100% duty cycle. The output voltage
will then be determined by the input voltage minus the IR
voltage drop across the external P-channel MOSFET,
sense resistor, and the inductor.
Undervoltage Lockout
A precision undervoltage lockout shuts down the LTC1773
when VIN drops below 2.5V, making it ideal for single
lithium-ion battery applications. In shutdown, the LTC1773
draws only several microamperes, which is low enough to
prevent deep discharge and possible damage to the lithiumion battery that’s nearing its end of charge. A 150mV
hysteresis ensures reliable operation with noisy supplies.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability by preventing
subharmonic oscillations. It works by internally adding a
ramp to the inductor current signal at duty cycles in excess
of 30%. This causes the internal current comparator to trip
earlier. The ITH clamp level is also reached earlier than
conditions in which the duty cycle is below 30%. As a
result, the maximum inductor peak current is lower for
VOUT/VIN > 0.3 than when VOUT/VIN < 0.3.
To compensate for this loss in maximum inductor peak
current during high duty cycles, the LTC1773 uses a
patent pending scheme that raises the ITH clamp level
(proportional to the amount of slope compensation) when
duty cycle is above 30%.
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APPLICATIONS INFORMATION
The basic LTC1773 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement and begins with the selection of RSENSE.
Once RSENSE is known, L can be chosen, followed by the
external power MOSFETs. Finally, CIN and COUT are selected.
average output current IMAX equal to the peak value less
half the peak-to-peak ripple current ∆IL.
RSENSE Selection for Output Current
Inductor Value Calculation
RSENSE is chosen based on the required output current.
The LTC1773 current comparator has a maximum threshold of 100mV/RSENSE. The current comparator threshold
sets the peak of the inductor current, yielding a maximum
The inductor selection will depend on the operating frequency of the LTC1773. The internal preset frequency is
550kHz, but can be externally synchronized up to 750kHz.
Allowing a margin for variations in the LTC1773 and
external component values yields:
RSENSE = 70mV/IMAX
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LTC1773
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APPLICATIONS INFORMATION
The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use
of smaller inductor and capacitor values. However, operating at a higher frequency generally results in lower
efficiency because of external MOSFET gate charge losses.
The inductor value has a direct effect on ripple current. The
ripple current, ∆IL, decreases with higher inductance or
frequency and increases with higher VIN or VOUT.
∆IL =
⎛ V ⎞
VOUT ⎜ 1– OUT ⎟
VIN ⎠
⎝
f L
1
( )( )
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 generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount are
available which do not increase the height significantly.
(1)
Accepting larger values of ∆IL allows the use of lower
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is 30% to 40% of IMAX. Remember,
the maximum ∆IL occurs at the maximum input voltage.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately 1/3
its original value. Lower inductor values (higher ∆IL) will
cause this to occur at lower load currents, which can cause
a dip in efficiency in the upper range of low current
operation. In Burst Mode operation, lower inductance
values will cause the burst frequency to increase.
Inductor Core Selection
Once the value for L is 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 losses 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
Power MOSFET and Schottky Diode Selection
Two external power MOSFETs must be selected for use
with the LTC1773: a P-channel MOSFET for the top (main)
switch, and an N-channel MOSFET for the bottom (synchronous) switch.
The peak-to-peak gate drive levels are set by the VIN
voltage. Therefore, for VIN > 5V, logic-level threshold
MOSFETs should be used. But, for VIN < 5V, sub-logic
level threshold MOSFETs (VGS(TH) < 3V) should be used.
In these applications, make sure that the VIN to the
LTC1773 is less than 8V because the absolute maximum
VGS rating of the majority of these sub-logic threshold
MOSFETs is 8V.
Selection criteria for the power MOSFETs include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS,
input voltage, maximum output current, and total gate
charge. When the LTC1773 is operating in continuous
mode the duty cycles for the top and bottom MOSFETs are
given by:
Main Switch Duty Cycle = VOUT/VIN
Synchronous Switch Duty Cycle = (VIN – VOUT)/VIN
The MOSFET power dissipations at maximum output
current are given by:
PMAIN =
VOUT
2
IMAX ) (1 + δ )RDSON +
(
VIN
K(VIN ) (IMAX )(C RSS )( f)
2
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LTC1773
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APPLICATIONS INFORMATION
PSYNC =
VIN – VOUT
2
IMAX ) (1 + δ )RDS(ON)
(
VIN
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 while the topside P-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. The synchronous
MOSFET losses are greatest at high input voltage or during
a short-circuit when the duty cycle in this switch is nearly
100%.
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.
Typical gate charge for the selected P-channel MOSFET
should be less than 30nC (at 4.5VGS) while the turn-off
delay should be less than 150ns. However, due to differences in test and specification methods of various MOSFET
manufacturers, the P-channel MOSFET ultimately should
be evaluated in the actual LTC1773 application circuit to
ensure proper operation.
A Schottky diode can be placed in parallel with the synchronous MOSFET to improve efficiency. It conducts
during the dead-time between the conduction of the two
power MOSFETs. This prevents the body diode of the
bottom MOSFET from turning on and storing charge
during the dead-time, which could cost as much as 1% in
efficiency. A 1A Schottky is generally a good size for 5A to
8A regulators due to the relatively small average current.
Larger diodes result in additional transition losses due to
their larger junction capacitance. The diode may be omitted if the efficiency loss can be tolerated.
CIN Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
12
VOUT (VIN – VOUT )]
[
CIN required IRMS ≅ IMAX
VIN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is
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
2000 hours of life. This makes it advisable to further derate
the capacitor, or 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 manufacturer if there is any
question.
COUT Selection
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR requirement is satisfied the capacitance is adequate for filtering.
The output ripple (∆VOUT) is determined by:
⎛
1 ⎞
∆VOUT ≅ ∆IL ⎜ ESR +
⎟
8fC OUT ⎠
⎝
where f = operating frequency, COUT = output capacitance
and ∆IL = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆IL increases
with input voltage. With ∆IL = 0.4IOUT(MAX) and allowing
for 2/3 of the ripple due to ESR, the output ripple will be
less than 50mV at max VIN assuming:
COUT required ESR < 2 RSENSE
COUT > 1/(8fRSENSE)
The first condition relates to the ripple current into the ESR
of the output capacitance while the second term guarantees that the output voltage does not significantly discharge during the operating frequency period due to ripple
current. The choice of using smaller output capacitance
increases the ripple voltage due to the discharging term
but can be compensated for by using capacitors of very
low ESR to maintain the ripple voltage at or below 50mV.
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The ITH pin OPTI-LOOP compensation components can be
optimized to provide stable, high performance transient
response regardless of the output capacitors selected.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance throughhole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR/size
ratio of any aluminum electrolytic at a somewhat higher
price. Once the ESR requirement for COUT has been met,
the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement.
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. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalum, available in case
heights ranging from 2mm to 4mm. Other capacitor types
include Sanyo OS-CON and POSCAP, Nichicon PL series,
Panisonic SP series and Sprague 593D and 595D series.
Consult the manufacturer for other specific recommendations.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
⎛ R2⎞
VOUT = 0.8V⎜ 1 + ⎟
⎝ R1⎠
Run/Soft-Start Function
The RUN/SS pin is a dual purpose pin that provides the
soft-start function and a means to shut down the LTC1773.
Soft-start reduces surge currents from VIN by gradually
increasing the internal current limit. Power supply sequencing can also be accomplished using this pin.
An internal 1.5µA current source charges up an external
capacitor CSS. When the voltage on RUN/SS reaches 0.7V
the LTC1773 begins operating. As the voltage on RUN/SS
continues to ramp from 0.7V to 1.8V, the ITH clamp is also
ramped at a proportionally linear rate. Depending on the
external RSENSE used, the peak inductor current, and thus
the internal current limit, rises with the RUN/SS voltage.
The output current thus ramps up slowly, charging the
output capacitor. If RUN/SS has been pulled all the way to
ground, there will be a delay before the current starts
increasing and is given by:
t DELAY =
(
)
0.7 C SS
= 0.47s / µF C SS
1.5µA
Pulling the RUN/SS pin below 0.4V puts the LTC1773 into
a low quiescent current shutdown mode (IQ < 10µA). This
pin can be driven directly from logic as shown in Figure 4.
Diode D1 in Figure 4 reduces the start delay but allows CSS
to ramp up slowly providing the soft-start function. This
diode can be deleted if soft-start is not needed.
3.3V OR 5V
RUN/SS
RUN/SS
D1
(2)
CSS
CSS
The external resistive divider is connected to the output as
shown in Figure 3, allowing remote voltage sensing.
1773 F04
Figure 4. RUN/SS Pin Interfacing
0.8V ≤ VOUT ≤ 8.5V
Auxiliary Winding Control Using SYNC/FCB Pin
R2
VFB
R1
LTC1773
GND
1773 F03
Figure 3. Setting the LTC1773 Output Voltage
The SYNC/FCB pin can be used as a secondary feedback
to provide a means of regulating a flyback winding output.
When this pin drops below its ground referenced 0.8V
threshold, continuous mode operation is forced. In continuous mode, the P-channel main and N-channel synchronous switches are switched continuously regardless
of the load on the main output.
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Synchronous switching removes the normal limitation
that power must be drawn from the inductor primary
winding in order to extract power from auxiliary windings.
With continuous synchronous operation, power can be
drawn from the auxiliary windings without regard to the
primary output load.
The secondary output voltage is set by the turns ratio of the
transformer in conjunction with a pair of external resistors
returned to the SYNC/FCB pin as shown in Figure 5. The
secondary regulated voltage, VSEC, in Figure 5 is given by:
⎛ R4 ⎞
VSEC ≅ (N + 1)VOUT − VDIODE > 0.8V⎜ 1 + ⎟
⎝ R3 ⎠
where N is the turns ratio of the transformer and VOUT is
the main output voltage sensed by VFB.
VIN
LTC1773
R4
TG
VSEC
+
L1
1:N
1µF
VOUT
SYNC/FCB
R3
SW
+
BG
COUT
1773 F05
Figure 5. Secondary Output Loop Connection
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1773 circuits: VIN quiescent current, external
power MOSFET gate charge current, I2R losses, and
topside MOSFET transition losses.
1. The VIN quiescent current is due to the DC bias current
as given in the electrical characteristics, it excludes
MOSFET driver and control currents. VIN current results
in a small loss which increases with VIN.
2. The external MOSFET gate charge current results from
switching the gate capacitance of the external power
MOSFET switches. Each time the gate is switched from
high to low to high again, a packet of charge dQ moves
from VIN to ground. The resulting dQ/dt is the current
out of VIN; it is typically larger than the DC bias current.
In continuous mode, IGATECHG = f(QT + QB) where QT
and QB are the gate charges of the external main and
synchronous switches. Both the DC bias and gate
charge losses are proportional to VIN and thus their
effects will be more pronounced at higher supply voltages.
3. I2R losses are calculated from the resistances of the
external RSENSE, the external power MOSFETs (RSW)
and the external inductor (RL). In continuous mode, the
average output current flowing through inductor L is
“chopped” between the main switch and the synchronous switch. Thus, the series resistance looking into
the SW pin from L is a function of both top and bottom
MOSFET RDS(ON) and the duty cycle (DC), as follows:
RSW = (RDS(ON)TOP +RSENSE) • DC + RDS(ON)BOT • (1 – DC)
Efficiency Considerations
The 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. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
The RDS(ON) for both the top and bottom MOSFETs can
be obtained from the MOSFET manufactures’s
datasheets. Thus, to obtain I2R losses, simply add RSW
and RL together and multiply their sum by the square of
the average output current.
4. Transition losses apply to the topside MOSFET and
increase when operating at high input voltages and
higher operating frequencies. Transition losses can be
estimated from:
Transition Loss = 2(VIN)2IO(MAX)CRSS(f)
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Other losses including CIN and COUT ESR dissipative
losses, 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 load current. When
a load step occurs, VOUT immediately 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, which generates a feedback error signal. The
regulator loop then returns VOUT to its steady-state value.
During this recovery time, VOUT can be monitored for
overshoot or ringing. OPTI-LOOP compensation allows
the transient response to be optimized over a wide range
of output capacitance and ESR values. The availability of
the ITH pin not only allows optimization of control loop
behavior but also provides a DC coupled and an AC filtered
closed-loop response test point. The DC step, rise time
and settling at this test point reflects the closed loop
response. Assuming a predominantly second order system, phase margin and/or damping factor can be 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 0.5 to 2 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 feedback factor gain and phase. An output current
pulse of 20% to 100% of full load current having a rise time
of 1µs to 10µs 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 may not be within the bandwidth of the
feedback loop, so the standard second order overshoot/
DC ratio cannot be used to determine phase margin. 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 shift 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. For a detailed explanation of optimizing the
compensation components, including a review of control
loop theory, refer to Application Note 76.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25)(CLOAD).
Thus a 10µF capacitor would require a 250µs rise time,
limiting the charging current to about 200mA.
Minimum On-Time Considerations
Minimum on-time, tON(MIN), is the smallest amount of
time that the LTC1773 is capable of turning the top
MOSFET on and off again. It is determined by internal
timing delays and the gate charge required to turn on the
top MOSFET. The minimum on-time for the LTC1773 is
about 250ns. Low duty cycle and high frequency synchronous applications may approach this minimum on-time
limit and care should be taken to ensure that:
t ON(MIN)<
VOUT
f • VIN
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1773 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple current and ripple voltage will increase.
If an application can operate close to the minimum ontime limit, an inductor must be chosen that has low
enough inductance to provide sufficient ripple amplitude
to meet the minimum on-time requirement. As a general
rule, keep the inductor ripple current equal or greater than
30% of the IOUT(MAX) at VIN(MAX).
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PC Board Layout Checklist
Design Example
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1773. These items are also illustrated graphically in
the layout diagram of Figure 6. Check the following in your
layout:
As a design example, assume the LTC1773 is used in a
single lithium-ion battery powered cellular phone application. The VIN will be operating from a maximum of 4.2V
down to about 2.7V. The load current requirement is a
maximum of 2A but most of the time it will be on standby
mode, requiring only 2mA. Efficiency at both low and high
load currents is important. Output voltage is 2.5V. With
this information we can calculate RSENSE to be around
33mΩ. For the inductor L, using equation (1),
1) Are the signal and power grounds segregated? The
LTC1773 signal ground consists of the resistive divider,
the compensation network and CSS. The power ground
consists of the (–) plate of CIN, the (–) plate of COUT, the
source of the external synchronous NMOS, and Pin 5 of
the LTC1773. The power ground traces should be kept
short, direct and wide. Connect the synchronous
MOSFETs source directly to the input capacitor ground.
2) Does the VFB pin connect directly to the feedback
resistors? The resistive divider of R1 and R2 must be
connected between the (+) plate of COUT and signal ground.
Be careful locating the feedback resistors too far away
from the LTC1773. The VFB line should not be routed close
to any other nodes with high slew rates.
3) Does the (+) terminal of CIN connect to VIN as closely as
possible? This capacitor provides the AC current to the
external power MOSFETs.
4) Keep the switching nodes SW, TG and BG away from
sensitive small-signal nodes, especially from the voltage
and current sensing feedback pins.
L=
⎛ V ⎞
VOUT ⎜ 1 – OUT ⎟
VIN ⎠
⎝
f ∆IL
1
( )( )
(3)
Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 800mA and
f = 550kHz in equation (3) gives:
L=
⎛ 2.5V ⎞
2.5V
⎟ = 2.3µH
⎜1–
550kHz (800mA) ⎝ 4.2V ⎠
A 2.5µH inductor works well for this application. For good
efficiency choose a 4A inductor with less than 0.1Ω series
resistance.
CIN will require an RMS current rating of at least 1A at
temperature and COUT will require an ESR of less than
0.066Ω. In most applications, the requirements for these
capacitors are fairly similar.
CC2
+
CC1
RC
1
ITH
SW
10
RSENSE
9
RUN/SS SENSE–
LTC1773
8
3
VIN
SYNC/FCB
+
2
CSS
R1
4
5
VFB
GND
TG
BG
D1
CIN
VIN
Q1
7
Q2
6
–
R2
–
+
COUT
L1
VOUT
+
BOLD LINES INDICATE
HIGH CURRENT PATHS
1773 F06
Figure 6. LTC1773 Layout Diagram
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For the selection of the external MOSFETs, the RDS(ON)
must be guaranteed at 2.5V since the LTC1773 has to
operate down to 2.7V. This requirement can be met by the
Si9801DY.
For the feedback resistors, choose R1 = 80.6k. R2 can then
be calculated from equation (2) to be:
⎞
⎛V
R2 = ⎜ OUT – 1⎟ R1 = 171k; use 169k
⎠
⎝ 0.8
Figure 7 shows the complete circuit along with its efficiency curve.
33pF
2.7V ≤ VIN ≤ 4.2V
30k
200pF
1
ITH
SW
2
0.1µF
–
10
9
RSENSE
0.033Ω
RUN/SS SENSE
LTC1773
3
8
VIN
VIN
SYNC/FCB
4
5
TG
VFB
BG
GND
L1
2.5µH
7
VOUT
2.5V
2A
+
6
+
Si9801DY
169k
1%
80.6k
1%
CIN: SANYO POSCAP 6TPA150M
COUT: AVX TPSD227M006R0100
L1: CDRH5D28
RSENSE: IRC LR1206-01-R033-F
CIN
150µF
6.3V
COUT
220µF
6.3V
1773 F07a
Figure 7. Single Lithium-Ion to 2.5V/2A Regulator
Efficiency Curve for Figure 7
100
VOUT = 2.5V
EFFICIENCY (%)
95
VIN = 3.3V
90
85
80
75
70
1
10
100
1000
OUTPUT CURRENT (mA)
5000
1773 F1b
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33pF
2.7V ≤ VIN ≤ 8.4V
200pF
1
30k
2
0.1µF
10
SW
SYNC/FCB
VIN
VFB
TG
5
+
L1
5µH
1:1
8
VOUT2
5V
100mA
22µF
6.2V
422k
VOUT1
2.5V
2A
7
6
BG
GND
RSENSE
0.033Ω
9
RUN/SS SENSE–
3
4
MBR0530LT1
LTC1773
ITH
169k
1%
+
Si9801DY
80.6k
1%
+
80.6k
COUT
220µF
6.3V
L1: COILTRONICS CTX5-4/BH ELECTRONICS 511-0033
RSENSE: IRC LR1206-01-R033-F
CIN: SANYO POSCAP 10TPA100M
COUT: AVX TPSD227M006R0100
CIN
100µF
10V
1773 TA01
Figure 8. Dual Output 2.5V/2A and 5V/100mA Application
33pF
2.7V ≤ VIN ≤ 5.5V
30k
200pF
1
2
0.1µF
VIN
3
4
5
LTC1773
ITH
SW
RUN/SS SENSE–
SYNC/FCB
VIN
VFB
TG
GND
BG
10
RSENSE
0.015Ω
9
Si9803DY
8
VOUT
2.5V
5A
L1
2.8µH
7
+
Si9804DY
6
+
169k
1%
80.6k
1%
CIN: SANYO POSCAP 6TPA150M
COUT: AVX TPSD227M006R0100
CIN
150µF
6.3V
COUT
220µF
6.3V
×3
L1: TOKO D104C 919AS-2R8M
RSENSE: DALE WSL-2010
1773 TA02
Figure 9. Single Lithium-Ion to 2.5V/5A Regulator
Efficiency Curve for Figure 9
100
95
VIN = 3.3V
EFFICIENCY (%)
90
VIN = 5V
85
80
75
70
65
60
1
10
100
IOUT (mA)
1000
10000
1773 • G17
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33pF
2.7V ≤ VIN ≤ 4.2V
30k
200pF
1
2
0.1µF
VIN
3
4
5
LTC1773
ITH
SW
RUN/SS SENSE
–
SYNC/FCB
VIN
VFB
TG
BG
GND
10
RSENSE
0.025Ω
9
Si9803DY
8
L1
2µH
47µF
7
VOUT
3.3V
1A
+
+
6
Si9804DY
+
249k
1%
80.6k
1%
CIN
150µF
6.3V
COUT
220µF
6.3V
L1
2µH
CIN: SANYO POSCAP 6TPA150M
COUT: AVX TPSD227M006R0100
L1: COILTRONICS CTX2-4/BH ELECTRONICS 511-1010
RSENSE: IRC LR1206-01-R033-F
1773 TA03
Figure 10. Single Lithium-Ion to 3.3V/1A Synchronous Zeta Converter
Efficiency Curve for Figure 10
100
VOUT = 3.3V
90
VIN = 5V
VIN = 4V
EFFICIENCY (%)
80
VIN = 3.3V
70
VIN = 2.7V
60
50
40
30
20
0.001
0.01
0.1
OUTPUT CURRENT (A)
1.0
1773 G18
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2.7V ≤ VIN ≤ 4.2V
47pF
30k
1
220pF
LTC1773
ITH
2
RUN/SS SENSE
4
SYNC/FCB
8
VIN
VFB
TG
GND
BG
5
100pF
RSENSE
0.05Ω
9
–
0.1µF 3
750kHz CLK
10
SW
VOUT
2.5V
1A
L1
3µH
7
CIN
47µF
6.3V
6
169k
1%
4.7µF
6.3V
Si6803DQ
+
100pF
80.6k
1%
+
COUT
47µF
6.3V
0.1µF
CIN, COUT: SANYO POSCAP 6TPA47M
L1: SUMIDA CDRH5D28 3R0
RSENSE: IRC LR1206-01-R050-J
1773 TA06
Figure 11. 750kHz Single Lithium-Ion to 2.5V/1A Regulator
VOUT3
2.5V
150mA
8
LT1762-2.5
IN
1
OUT
2
1µF
10µF
SENSE
0.01µF
5
3
SHDN
VOUT1
1.8V
6A
BYP
GND
VOUT2
3.3V
1A
47pF
3.3V ≤ VIN ≤ 6V
220pF
30k
1
2
0.1µF
3
4
100pF
5
LTC1773
ITH
SW
RUN/SS SENSE–
SYNC/FCB
VIN
VFB
TG
GND
BG
100k
1%
100pF
80.6k
1%
CIN: PANASONIC SPECIAL POLYMER
COUT: KEMET T510687K004AS
T1: BH ELECTRONICS 510-1007
RSENSE: IRC LR2512-01-R010-J
CSEC: TAIYO YUDEN LMK432F476ZM
10
RSENSE
0.01Ω
9
MBRM120T3
+
8
CSEC
47µF
6.3V
+
7
0.1µF
249k
1%
T1
2.44µH
1:1
6
CIN
150µF
6.3V
80.6k
1%
Si7540DP
D2*
MBRS340T3
+
COUT
680µF
4V
×2
1773 TA07
*NOTE: D2 NOT NECESSARY.
IF REMOVED, EFFICIENCY DROPS BY 1%
Figure 12. Triple Output 1.8V/6A, 2.5V/150mA,and 3.3V/1A Application
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2.7V ≤ VIN ≤ 6V
47pF
30k
1
220pF
2
0.1µF
3
4
5
100pF
LTC1773
ITH
10
SW
RUN/SS SENSE–
VFB
TG
GND
BG
D1
MMSD914T1
8
VIN
SYNC/FCB
RSENSE
0.068Ω
9
VOUT
2V
800mA
L1
4.2µH
7
CIN
47µF
6.3V
6
118k
1%
4.7µF
6.3V
Si9801DY
100pF
80.6k
1%
+
D2*
MBR0530LT1
0.1µF
+
COUT
47µF
6.3V
*NOTE: D2 NOT NECESSARY.
IF REMOVED, EFFICIENCY DROPS BY 1%
CIN, COUT: SANYO POSCAP 6TPA47M
L1: SUMIDA CDRH5D28 4R2
RSENSE: IRC LR1206-01-R068-F
1773 TA05
Figure 13. Single Lithium-Ion to 2V/800mA Regulator with
Current Foldback
2.7V ≤ VIN ≤ 6V
47pF
30k
220pF
0.1µF
1
2
3
4
100pF
5
LTC1773
ITH
SW
RUN/SS SENSE–
SYNC/FCB
VIN
VFB
TG
GND
BG
100k
1%
10
RSENSE
0.01Ω
9
8
VOUT
1.8V
7A
L1
1µH
7
4.7µF
6.3V
Si7540DP
+
100pF
D2*
MBRS340T3
CIN: PANASONIC SPECIAL POLYMER
COUT: KEMET T510687K004AS
L1: TOKO TYPE D104C 919AS-1RON
RSENSE: IRC LR2512-01-R010-J
CIN
150µF
6.3V
6
0.1µF
80.6k
1%
+
COUT
680µF
4V
×2
*NOTE: D2 NOT NECESSARY.
IF REMOVED, EFFICIENCY DROPS BY 1%
1773 TA08
Figure 14. 3.3V to 1.8V/7A Regulator
1773fb
18
LTC1773
U
TYPICAL APPLICATIONS
33pF
4.5V ≤ VIN ≤ 5.5V
200pF
1
30k
2
0.1µF
VIN
3
4
5
LTC1773
ITH
SW
–
RUN/SS SENSE
SYNC/FCB
VIN
VFB
TG
BG
GND
10
9
RSENSE
0.04Ω
8
L1
2.5µH
7
VOUT
1.8V
2A
CIN
47µF
10V
6
COUT
47µF
10V
Si9942DY
100k
1%
80.6k
1%
CIN, COUT: TAIYO YUDEN LMK550BJ476MM
L1: CDRH5D28
RSENSE: IRC LR1206-01-R040-F
1773 TA09
Figure 15. 5V to 1.8V/2A Regulator with Ceramic Capacitors
U
PACKAGE DESCRIPTIO
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
10 9 8 7 6
0.50
0.305 ± 0.038
(.0197)
(.0120 ± .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.497 ± 0.076
(.0196 ± .003)
REF
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
1 2 3 4 5
0° – 6° TYP
GAUGE PLANE
0.86
(.034)
REF
1.10
(.043)
MAX
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
SEATING
PLANE
0.18
(.007)
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
0.127 ± 0.076
(.005 ± .003)
MSOP (MS) 0603
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
1773fb
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.
19
LTC1773
U
TYPICAL APPLICATIO
33pF
30k
4.5V ≤ VIN ≤ 5.5V
200pF
1
2
0.1µF
VIN
3
4
5
LTC1773
ITH
SW
RUN/SS SENSE–
SYNC/FCB
VIN
VFB
TG
GND
BG
10
9
L1
10µH
3:1
8
+
Si2302DS
6
COUT2
150µF
6.3V
D1
+
169k
1%
CIN
150µF
6.3V
VOUT1
2.5V
1A
7
Si6801DY
40.2k
1%
+
VOUT2
3.3V
500mA
RSENSE
0.050Ω
249k
1%
47k
COUT1
150µF
6.3V
0.1µF
CIN, COUT1, COUT2: SANYO POSCAP 6TPA150M
RSENSE: IRC LR1206-01-R050-F
D1: BAS16
1773 TA04
Figure 16. Dual Output Synchronous Buck Converter
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1622
Low Voltage Current Mode Step-Down DC/DC Controller
VIN: 2V to 10V, 550kHz, Burst Mode Operation, Synchronizable
LTC1627/LTC1707 Low Voltage, Monolithic Synchronous Step-Down Regulator
Low Supply Voltage Range: 2.65V to 8V, IOUT to 0.5A
LTC1735
High Efficiency Synchronous Step-Down Switching Controller
Burst Mode Operation, 16-Pin Narrow SO, Fault Protection
LTC1735-1
High Efficiency Synchronous Step-Down Switching Controller
Output Fault Protection 16-Pin GN, Burst Mode Operation,
Power Good
LTC1771
Low Quiescent Current Step-Down DC/DC Controller
VIN: 2.8V to 18V, 10µA IQ, MS8 Package
LTC1772/B
SOT-23 Low Voltage Step-Down Controller
6-Pin SOT-23, 2V ≤ VIN ≤ 10V, 550kHz
LTC1778
Wide Operating Range/Step-Down Controller, No RSENSE
VIN up to 36V, Current Mode, Power Good
LTC1779
SOT-23 Current Mode Step-Down Converter
250mA Output Current, 2.5V ≤ VIN ≤ 9.8V, Up to 94% Efficiency
LTC1877
High Efficiency Monolithic Synchronous Step-Down Regulator
VIN from 2.65V to 10V, 10µA IQ, 550kHz, IOUT to 600mA,MS8
LTC1878
High Efficiency Monolithic Synchronous Step-Down Regulator
VIN from 2.65V to 7V, 10µA IQ, 550kHz, IOUT to 600mA,MS8
LTC3404
1.4MHz Monolithic Synchronous Step-Down Regulator
Up to 95% Efficiency, IOUT = 600mA at VIN = 3.3V
No Schottky Diode Required, 8-Lead MSOP
1773fb
20
Linear Technology Corporation
LT 1106 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
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