LINER LTC3458L 1.7a, 1.5mhz synchronous step-up dc/dc converter with output disconnect Datasheet

LTC3458L
1.7A, 1.5MHz Synchronous
Step-Up DC/DC Converter
with Output Disconnect
DESCRIPTIO
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FEATURES
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High Efficiency: Up to 95%
Inrush Current Limiting and Output Disconnect
Programmable Output Voltages up to 6V
1.5V to 6V Input Range
Programmable/Synchronizable Fixed Frequency
Operation up to 1.5MHz
Programmable Automatic Burst Mode® Operation
Current Mode Control with Programmable Soft-Start
Period and Peak Current Limit
0.2Ω N-Channel and 0.3Ω P-Channel Switches
Ultralow Quiescent Currents: 12μA Sleep, <1μA in
Shutdown
12-Pin 4mm × 3mm Thermally Enhanced DFN
Package
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APPLICATIO S
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Point-of-Load Regulators
USB VBUS Power
Portable GPS
MP3 Players
The LTC®3458L is a high efficiency, current mode, fixed
frequency, step up DC/DC converter with true output
disconnect and inrush current limiting. The LTC3458L is
rated for up to 6V output and includes a 0.2Ω N-channel
MOSFET switch and a 0.3Ω P-channel MOSFET synchronous rectifier. The LTC3458L is well suited for battery
powered applications and includes programmable output
voltage, switching frequency and loop compensation. The
oscillator frequency can be set up to 1.5MHz or synchronized to an external clock.
Quiescent current is only 12μA during Burst Mode operation maximizing battery life in portable applications. The
Burst Mode current threshold, peak current limit, and softstart are externally programmable. Other features include
<1μA shutdown current, antiringing control, and thermal
limit. The LTC3458L is available in a low profile (0.75mm),
4mm × 3mm 12-pin DFN package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All
other trademarks are the property of their respective owners. Burst Mode is a registered
trademark of Linear Technology Corporation. Protected by U.S. Patents including 5481178,
6580258, 6304066, 6127815, 6498466, 6611131.
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TYPICAL APPLICATIO
Li-Ion to 5V at 1MHz
7.4μH
Li-Ion to 5VOUT
SW
VIN
LTC3458L
2.2μF
10pF
ON OFF
SHDN
FB
SYNC
COMP
RT
33k
SS
0.01μF
ILIM
200k
84.5k
1M
324k
0.01μF
95
VOUT
5V
750mA
VOUT
1000
4.2VIN
3.1VIN
90
100
3.6VIN
85
80
10
POWER
LOSS
75
70
22μF
X5R
10pF
1
3.6 VIN POWER LOSS (mW)
GND/PGND
EFFICIENCY (%)
Li Ion
3.1V TO 4.2V
100
65
BURST
133k
60
560pF
0.1
1
100
10
LOAD CURRENT (mA)
0.1
1000
3458L TA01a
3458L TA01b
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ABSOLUTE
AXI U RATI GS
PIN CONFIGURATION
(Note 1)
TOP VIEW
VIN, SS, SYNC Voltages ................................. –0.3 to 7V
BURST, SHDN, VOUT Voltages ....................... –0.3 to 7V
Operating Temperature Range
(Notes 2, 3) .........................................–40°C to 85°C
Storage Temperature Range ..................–65°C to 125°C
SW Voltage
DC ........................................................... –0.3V to 7V
Pulsed <100ns ........................................ –0.3V to 8V
SW
1
12 VOUT
VIN
2
11 BURST
SYNC
3
10 SS
13
SHDN
4
9
GND
ILIM
5
8
COMP
RT
6
7
FB
DE PACKAGE
12-LEAD (4mm × 3mm) PLASTIC DFN
EXPOSED PAD IS PGND (PIN 13),
MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 45°C/W
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3458LEDE#PBF
LTC3458LEDE#TRPBF
3458L
12-Lead (4mm x 3mm) Plastic DFN
–40ºC to 85ºC
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC for information on nonstandard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 5V, RT = 200k, unless otherwise noted.
PARAMETER
CONDITIONS
Minimum VIN Operating Voltage
TA = 0°C to 85°C
TA = –40°C to 0°C
●
Output Voltage Adjust Range
Feedback Voltage
MIN
0°C to 85°C, VOUT = 3.3V
–40°C to 0°C
TYP
MAX
1.4
1.4
1.5
1.7
V
V
6
V
1.225
1.25
1.25
V
V
2
1.21
1.20
UNITS
Undervoltage (Exit Burst Mode Operation)
Below Feedback Voltage
–4
Feedback Input Current
VFB = 1.23V
1
50
nA
%
Quiescent Current – Burst Mode Operation
VIN Current at 3.3V
VOUT Current at 5V
12
5
30
10
μA
μA
Quiescent Current – Shutdown
VIN Current at 3.3V
VOUT Current at 0V
0.5
1
1
3
μA
μA
Quiescent Current – Active
VIN Current Switching
1
3
mA
NMOS Switch Leakage
●
0.05
5
μA
PMOS Switch Leakage
●
0.05
5
μA
NMOS Switch On Resistance
VOUT = 5V
0.2
Ω
PMOS Switch On Resistance
VOUT = 5V
0.3
Ω
Fixed NMOS Current Limit
RILIM = 84.5k
●
1.7
2.1
A
Maximum Duty Cycle
VIN = 3.3V, fOSC = 1MHz
●
80
90
●
Minimum Duty Cycle
Frequency Accuracy
RT = 200k
●
%
0
0.85
1
1.15
%
MHz
Error Amplifier Transconductance
60
μA/V
Error Amplifier Source Current
7
μA
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LTC3458L
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 5V, RT = 200k, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
Error Amplifier Sink Current
●
SYNC Input Low
●
SHDN Input High
●
SHDN Input Low
●
BURST Mode Peak Current (IBURSTPEAK)
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ILIMIT, IBURSTPEAK, IZERO Currents
Current Limit Accuracy
V
400
CURRENT (μA)
CURRENT (A)
CURRENT (mA)
IBURSTPEAK
2.05
V
IVIN
10
5
1.95
200
1.10
15
2.15
600
A
VIN = 3.3V
VOUT = 5V
1800
800
0.55
Burst Mode Quiescent Current
RILIMIT = 84.5k
1000
V
20
ILIMIT
VOUT = 5V
L = 7.4μH
RLIM = 84.5k
0.3
(TA = 25°C unless otherwise specified)
2.25
1600
V
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3: The LTC3458LE 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.
TYPICAL PERFOR A CE CHARACTERISTICS
2200
0.35
1.25
RILIM = 84.5k
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: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
1200
μA
1.5
BURST Threshold Voltage
1400
UNITS
7
SYNC Input High
2000
MAX
IVOUT
IZERO
0
–200
2.0
2.5
3.0
3.5 4.0
VIN (V)
4.5
5.0
1.85
–45 –30 –15
5.5
0 15 30 45 60
TEMPERATURE (°C)
75
3458L G01
3458L G02
Typical Burst Mode Threshold and
Hysteresis vs RBURST
Maximum Load Current in Burst
60
500
INTO
BURST
450
5VOUT
200
RT (kΩ)
CURRENT (mA)
CURRENT (mA)
100
150
100
400
350
300
250
40
200
50
20
0
50
85
550
3.3VOUT
OUT OF
BURST
60
Oscillator Programming Resistor
250
120
35
10
TEMPERATURE (°C)
600
RILIM = 84.5k
140
–15
3458L G03
300
160
80
0
–40
90
150
100
150
200
RBURST (kΩ)
250
300
3458L G04
0
1.5
2.0
2.5
3.0
VIN (V)
3.5
4.0
4.5
3458L G05
100
400
600
1000 1200
1400
800
OSCILLATOR FREQUENCY (kHz)
3458L G06
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TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Accuracy
Efficiency vs Frequency
0.5
VIN = 3.3V
VOUT = 5V
RT = 200k
93
EFFICIENCY (%)
1.03
FREQUENCY (MHz)
N-Channel and P-Channel RDS(ON)
95
1.01
0.99
P-CHANNEL
0.3
RDS(ON) (Ω)
1.05
(TA = 25°C unless otherwise specified)
91
89
0.2
N-CHANNEL
0.1
87
0.97
0.95
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
VIN = 3.3V
VOUT = 5V at 200mA
L = 7.4μH
85
500
90
700
1100
1300
900
FREQUENCY (kHz)
3458L G07
85
SYNC Pin Threshold
5VOUT
1.4
0.9
1.3
1000
VOLTAGE (V)
3.3VOUT
800
600
OPERATING
VOLTAGE (V)
MAX LOAD CURRENT (mA)
60
1.5
1.0
1200
0.8
0.7
SHUTDOWN
1.2
1.1
1.0
400
0.6
0.9
200
0
1.5 2.0
35
10
TEMPERATURE (°C)
3458L G09
SHDN Pin Threshold and
Hysteresis
1600
1.8 to 5.5VIN at 1MHz
RILIM = 84.5k
–15
3458L G08
Maximum Load Current
1400
0
–40
1500
2.5
3.0 3.5 4.0
VIN (V)
4.5
5.0
0.5
–45 –30 –15
5.5
0 15 30 45 60
TEMPERATURE (°C)
75
90
3458L G11
3458L G10
0.8
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
3458L G12
FB Voltage
1.25
VOLTAGE (V)
1.24
1.23
1.22
1.21
1.20
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
3458L G13
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TYPICAL PERFOR A CE CHARACTERISTICS
(TA = 25°C unless otherwise specified)
Fixed Frequency (FF)
Discontinuous Current
Fixed Frequency (FF)
Continuous Current
SW
1V/DIV
SW
1V/DIV
IL
200mA/DIV
GND
IL
100mA/DIV
0mA
0mA
VIN = 3.3V
VOUT = 5V
L = 7.4μH
500ns/DIV
VIN = 3.3V
VOUT = 5V
L = 7.4μH
3458L G14
Over-Current with 2A ILIMIT
500ns/DIV
3458L G15
Burst Mode Operation
VOUT
100mV/DIV
IL
0.5A/DIV
SW
5V/DIV
SW
2V/DIV
0mA
VIN = 2.25V
VOUT = 5V
L = 7.4μH
RILIM = 84.5k
1μs/DIV
IL
200mA/DIV
0mA
VIN = 3.3V
VOUT = 5V
L = 7.4μH
COUT = 22μF
CFF = 22pF
RILIM = 84.5k
3458L G16
Burst Mode Operation Close-Up
50μs/DIV
3458L G17
Soft-Start into 20Ω Load
VOUT
100mV/DIV
VOUT
2V/DIV
VIN
2V/DIV
SW
5V/DIV
IL
200mA/DIV
0mA
VIN = 3.3V
3458L G18
2μs/DIV
VOUT = 5V
L = 7.4μH
COUT = 22μF
CFF = 22pF*
RILIM = 84.5k
*CFF: FEED FORWARD CAPACITOR CONECTED BETWEEN VOUT AND FB.
SS
200mV/DIV
IL
200mA/DIV
VIN = 3.3V
VOUT = 5V
L = 7.4μH
CSS = 0.1μF
2ms/DIV
3458L G19
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TYPICAL PERFOR A CE CHARACTERISTICS
(TA = 25°C unless otherwise specified)
Sync Operation at 1.33MHz
FF Mode 100-300mA Load Step
SW
5V/DIV
VOUT
200mV/DIV
SYNC
2V/DIV
COMP
500mV/DIV
IL
200mA/DIV
IL
0.5A/DIV
0mA
0mA
VIN = 3.3V
VOUT = 5V
RT = 200k
500ns/DIV
VIN = 3.3V
VOUT = 5V
RZ = 33K
CC1 = 270pF
CC2 = 10pF
COUT = 22μF
3458L G20
200μs/DIV
3458L G21
L = 10μH
F = 1MHz
Auto Mode 10mA to 100mA Load
Step
Burst Mode Operation 10mA to
50mA Load Step
VOUT
200mV/DIV
VOUT
200mV/DIV
BURST
500mV/DIV
50mA
LOAD
10mA
BURST
1V/DIV
100mA
LOAD
10mA
IL
200mA/DIV
IL
200mA/DIV
VIN = 3.3V
VOUT = 5V
L = 10μH
COUT = 22μF
RILIM = 110k
200μs/DIV
500μs/DIV
VIN = 3.3V
VOUT = 5V
L = 10μH
CBURST = 0.015μF
RBURST = 133k
RILIM = 110k
3458L G22
10mA to 200mA Load Step
Showing UV Trip
3458L G23
Forced BURST to FF Mode
Switch with 50mA Load
VOUT
200mV/DIV
VOUT
200mV/DIV
– 4%
FIXED
FREQUENCY
BURST
IL
200mA/DIV
200μs/DIV
VIN = 3.3V
VOUT = 5V
L = 10μH
CBURST = 0.015μF
RBURST = 133k
RILIM = 110k
3458L G24
IL
500mA/DIV
200μs/DIV
3458L G25
VIN = 3.3V
VOUT = 5V
L = 10μH
RZ = 33k
CC1 = 270pF
CC2 = 10pF
COUT = 22μF
RILIM = 110k
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LTC3458L
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SW (Pin 1): Switch Pin for Inductor Connection. During
discontinuous conduction mode an antiring resistor connects SW to VIN to reduce noise.
VIN (Pin 2): Input Supply Pin. Connect this to the input
supply and decouple with 1μF minimum.
SYNC (Pin 3): Oscillator Synchronization Pin. A clock
pulse width of 100ns to 2μs is required to synchronize the
internal oscillator. This pin is disabled when grounded.
SHDN (Pin 4): Shutdown Pin. Grounding this pin shuts
down the IC. Connect to >1.25V to enable.
ILIM (Pin 5): Adjustable Peak Current Limit. Connect a
resistor from ILIM to GND to program the peak inductor
current according to the following formula:
180
ILIMIT =
RILIM
where ILIMIT is in amps and RILIM is in kΩ.
RT (Pin 6): Connect a resistor to ground to program the
oscillator frequency, according to the formula:
1
0.2 + 0.004 • RT
where fOSC is in MHz and RT is in kΩ.
fOSC =
Oscillator frequency can be programmed between 400kHz
and 1.5MHz.
FB (Pin 7): Connect Resistor Divider Tap Here. The output
voltage can be adjusted from 2V to 6V. Feedback reference
voltage is typically 1.225V.
COMP (Pin 8): gm Error Amp Output. A frequency compensation network is connected from this pin to ground to
compensate the loop. See the section “Compensating the
Feedback Loop” for guidelines.
GND (Pin 9): Signal Ground Pin.
SS (Pin 10): Connect a capacitor between this pin and
ground to set soft-start period. 5μA of current is sourced
from SS during soft-start.
t(msec) = CSS (μF )• 200
BURST (Pin 11): Burst Mode Threshold Adjust Pin. A
resistor/capacitor combination from this pin to ground
programs the average load current at which automatic
Burst Mode operation is entered, according to the formula:
RBURST =
10
IBURST
where RBURST is in kΩ and IBURST is in amps.
C BURST =
C OUT • VOUT
10, 000
where CBURST(MIN) and COUT are in μF.
To force fixed frequency PWM mode, connect BURST to
VOUT through a 51k resistor.
VOUT (Pin 12): Output of the Synchronous Rectifier and
Internal Gate Drive Source for the Power Switches.
⎛ R1⎞
VOUT = 1.225⎜ 1 + ⎟
⎝ R2⎠
Exposed Pad (PGND) (Pin 13): Must be soldered to PCB
ground, for electrical contact and optimum thermal
performance.
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BLOCK DIAGRA
BURST
SW
VIN
11
1
2
VIN
3
OSC/SYNC
RT
6
MAX
DUTY
CLOCK
SYNC
ANTIRING
SLOPE
UNDER
BURST MODE
CONTROL
P-DRIVE
N-DRIVE
+
MODE
VBEST
IZERO
SW1
IZERO
DETECT
P-DRIVE
PWM
AND
BURST MODE
12 VOUT
N-DRIVE
P-DRIVE
DRIVE LOGIC
4%
UNDERVOLTAGE
UNDER
SLEEP
IPEAK
–
VSELECT
MODE
FIXED
FREQUENCY
BURST MODE
MUX
SLEEP
CONTROL
ICOMP/LIMIT_PEAK
IBURST_PEAK
SLOPE
PGND
SLEEP TO
ALL BLOCKS
BURST ACTIVE
–
7 FB
VIN
+
(DISABLED IN
BURST MODE)
PEAK CURRENT
COMPARATOR
ICOMP, ILIMIT,
IBURST_PEAK,
SLOPE COMP
ERROR AMPLIFIER/
BURST COMPARATOR
BIAS
CURRENTS
MODE
I_SENSE
UVLO
REFERENCE/
BIAS
TSD
VBEST
SOFT-START
THERMAL SD
N-DRIVE
SD
SDB
TO ALL BLOCKS
PGND
9
5
13
10
8
4
GND
ILIM
PGND
SS
COMP
SHDN
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APPLICATIO S I FOR ATIO
Detailed Description
LTC3458L Programmable Functions
The LTC3458L provides high efficiency, low noise power
for boost applications with output voltages up to 6V. The
true output disconnect feature eliminates inrush current,
and allows VOUT to go to zero during shutdown. The
current mode architecture with adaptive slope compensation provides ease of loop compensation with excellent
transient load response. The low RDS(ON), low gate charge
synchronous switches eliminate the need for an external
Schottky rectifier, and provide efficient high frequency
pulse width modulation (PWM) control. High efficiency is
achieved at light loads when Burst Mode operation is
entered, where the IC’s quiescent current is a low 12μA
typical on VIN. The LTC3458L is designed to provide
custom performance in a variety of applications with
programmable feedback, current limit, oscillator frequency,
soft-start, and Burst Mode threshold.
Current Limit (ILIMIT)/Peak Burst Current (IBURSTPEAK).
The programmable current limit circuit sets the maximum
peak current in the internal N-channel MOSFET switch.
This clamp level is programmed using a resistor to ground
on ILIM. In Burst Mode operation, the current limit is
automatically set to ~1/4 of the programmed current limit
for optimal efficiency. An 84.5k RILIM resistor is recommended in most applications unless a lower limit is
needed to prevent the external inductor from saturating.
ILIMIT =
180
R
I is in amps and R is in kΩ.
IBURSTPEAK ≈
1
• ILIM
4
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APPLICATIO S I FOR ATIO
Error Amp. The error amplifier is a transconductance type,
with its positive input internally connected to the 1.225V
reference, and its negative input connected to FB. A simple
compensation network is placed from COMP to ground.
Internal clamps limit the minimum and maximum error
amp output voltage for improved large signal transient
response. During sleep (in Burst Mode), the compensation pin is high impedance, however clamps limit the
voltage on the external compensation network, preventing
the compensation capacitor from discharging to zero
during the sleep time.
Oscillator. The frequency of operation is set through a
resistor from RT to ground. An internally trimmed timing
capacitor resides inside the IC. The oscillator frequency is
calculated using the following formula:
fOSC =
1
0.2 + 0.004 • RT
where fOSC is in MHz and RT is in kΩ
The oscillator can be synchronized with an external clock
applied to the SYNC pin. When synchronizing the oscillator, the free running frequency must be set to approximately 30% lower than the desired synchronized frequency.
Soft-Start. The soft-start time is programmed with an
external capacitor to ground on SS. An internal current
source charges it with a nominal 5μA. The voltage on the
SS pin (in conjunction with the external resistor on ILIM) is
used to control the peak current limit until the voltage on
the capacitor exceeds ~1V, at which point the external
resistor sets the peak current. In the event of a commanded shutdown, severe short-circuit, or a thermal
shutdown, the capacitor is discharged automatically.
t(msec) = CSS (μF) • 200
Current Sensing. Lossless current sensing converts the
peak current signal to a voltage to sum in with the internal
slope compensation. This summed signal is compared to
the error amplifier output to provide a peak current control
command for the PWM. The slope compensation in the IC
is adaptive to the input and output voltage, therefore the
converter provides the proper amount of slope compensation to ensure stability, but not an excess to cause a loss
of phase margin in the converter.
Output Disconnect and Inrush Limiting. The LTC3458L
is designed to allow true output disconnect by eliminating
body diode conduction of the internal P-channel MOSFET
rectifier. This allows V0UT to go to zero volts during
shutdown, drawing no current from the input source. It
also allows for inrush current limiting at turn-on, minimizing surge currents seen by the input supply. Note that to
obtain the advantages of output disconnect, there must
not be any external Schottky diodes connected between
SW and VOUT.
Shutdown. The part is shut down by pulling SHDN below
0.3V, and made active by pulling the pin above 1.25V. Note
that SHDN can be driven above VIN or VOUT, as long as it
is limited to less than 6V.
Synchronous Rectifier. To prevent the inductor current
from running away, the P-channel MOSFET synchronous
rectifier is only enabled when VOUT > (VIN + 0.25V).
Thermal Shutdown. If the die temperature reaches approximately 150°C, the part will go into thermal shutdown
and all switches will be turned off and the soft-start
capacitor will be reset. The part will be enabled again when
the die temperature has dropped by 10°C (nominal).
Zero Current Amplifier (IZERO). The zero current amplifier
monitors the inductor current to the output and shuts off
the synchronous rectifier once the current is below 50mA
typical, preventing negative inductor current.
Other LTC3458L Features and Functions
Antiringing Control. The antiringing control places a
resistor across the inductor to damp the ringing on SW pin
discontinuous conduction mode. The LC ringing
(L = inductor, CSW = Capacitance on SW pin) is low energy,
but can cause EMI radiation.
Burst Mode Operation
Burst Mode operation can be automatic or user controlled.
In automatic operation, the IC will automatically enter
Burst Mode operation at light load and return to fixed
frequency PWM mode for heavier loads. The user can
program the average load current at which the mode
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APPLICATIO S I FOR ATIO
transition occurs using a single resistor. During Burst
Mode operation, the oscillator is shut down, since the on
time is determined by the time it takes the inductor current
to reach a fixed peak current, and the off time is determined by the time it takes for the inductor current to return
to zero.
In Burst Mode operation, the IC delivers energy to the
output until it is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
12μA of quiescent current. In this mode the output ripple
voltage has a variable frequency component with load
current and will be typically 2% peak-to-peak. This maximizes efficiency at very light loads by minimizing switching and quiescent losses. Burst Mode ripple can be reduced slightly by using more output capacitance (22μF or
greater). This capacitor does not need to be a low ESR type
if low ESR ceramics are also used. Another method of
reducing Burst Mode ripple is to place a small feedforward capacitor across the upper resistor in the VOUT
feedback divider network.
During Burst Mode operation, COMP is disconnected
from the error amplifier in an effort to hold the voltage on
the external compensation network where it was before
entering Burst Mode operation. To minimize the effects of
leakage current and stray resistance, voltage clamps limit
the minimum and maximum voltage on COMP during
Burst Mode operation. This minimizes the transient experienced when a heavy load is suddenly applied to the
converter after being in Burst Mode operation for an
extended period of time.
For automatic operation, an RC network should be connected from BURST to ground. The value of the resistor
will control the average load current (IBURST) at which
Burst Mode operation will be entered and exited (there is
hysteresis to prevent oscillation between modes). The
equation given for the capacitor on BURST is for the
minimum value, to prevent ripple on the BURST pin from
causing the part to oscillate in and out of Burst Mode
operation at the current where the mode transition occurs.
RBURST =
10
C BURST =
C OUT • VOUT
10, 000
where CBURST(MIN) and COUT are in μF.
Note: the BURST pin only sources current based on
current delivered to VOUT through the P-channel MOSFET.
If current in the inductor is allowed to go negative (this can
occur at very light loads and high step-up ratios), the burst
threshold may become inaccurate, preventing the IC from
entering Burst Mode operation. For RBURST values greater
than 200k, a larger than recommended inductor value may
be needed to ensure positive inductor current and automatic Burst Mode operation.
In the event that a sudden load transient causes the voltage
level on FB to drop by more than 4% from the regulation
value, an internal pull-up is applied to BURST, forcing the
part quickly out of Burst Mode operation. For optimum
transient response when going between Burst Mode operation and PWM mode, Burst can be controlled manually
by the host. This way PWM mode can be commanded
before the load step occurs, minimizing output voltage
drop. Note that Burst Mode operation is inhibited during
start-up and soft-start.
Manual Control
For applications requiring fixed frequency operation at all
load currents, connect the BURST pin to VOUT through a
51k resistor. To force Burst Mode operation, ground the
BURST pin.
For applications where a large load step can be anticipated,
the circuit below can be used to reduce the voltage transient
on VOUT. Automatic operation is achieved when the external P-channel MOSFET is off and fixed frequency operation
is commanded when the external P-channel MOSFET is on.
In shutdown, the P-channel MOSFET should be off.
VIN
HIGH: AUTO MODE
LOW: FIXED FREQUENCY
P-CHANNEL
MOSFET
BURST
133k
0.01μF
IBURST
3458L FO1
where RBURST is in kΩ and IBURST is in amps.
Figure 1
3458lfa
10
LTC3458L
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APPLICATIO S I FOR ATIO
COMPONENT SELECTION
Some example inductor part types are:
Inductor Selection
The high frequency operation of the LTC3458L allows for
the use of small surface mount inductors. Since the
internal slope compensation circuit relies on the inductor’s
current slope and frequency, Table 1 should be used to
select an inductor value for a given frequency of operation
(± 25%). The recommended value will yield optimal transient performance while maintaining stable operation.
Inductor values larger than listed in Table 1 are permissible to reduce the current ripple.
Table 1. Recommended Inductor Values
Frequency
Inductor Value(μH)
1.5MHz
3.3 to 4.7
1.25MHz
4.7 to 6.8
1MHz
6.8 to 10
750kHz
10 to 15
500kHz
15 to 22
For high efficiency, choose an inductor with high frequency core material, such as ferrite, to reduce core
losses. The inductor should have low ESR (equivalent
series resistance) to reduce the I2R losses, and must be
able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents in the
1A to 3A region. To minimize radiated noise, use a
toroidal or shielded inductor. (Note that the inductance of
shielded types will drop more as current increases, and
will saturate more easily). See Table 2 for a list of inductor
manufacturers.
Table 2. Inductor Vendor Information
Supplier
Phone
Website
Coilcraft
(847) 639-6400
www.coilcraft.com
TDK
(847) 803-6100
www.component.tdk.com
Murata
Sumida
USA: (814) 237-1431
(800) 831-9172
USA: (847) 956-0666
Japan: 81-3-3607-5111
www.murata.com
www.japanlink.com/sumida
COEV
(800) 227-7040
www.coev.net
Toko
..
Wurth
(847) 297-0070
www.tokoam.com
(202) 785-8800
www.we-online.com
Coilcraft: MSS7341 and MSS5131 Series
TDK: RLF5018T and SLF7045 Series
Murata: LQH4C and LQN6C Series
Sumida: CDRH4D28 and CDRH6D28 Series
COEV: DQ7545 Series
TOKO:
D62CB and D63LCB Series
..
WURTH: WE-PD2 Series
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The max ripple due
to charge is given by:
VRBULK =
IP • VIN
C OUT • VOUT • f
where IP = peak inductor current and f = switching
frequency.
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is given by:
VRCESR = IP • CESR
where CESR = Capacitor Series Resistance.
The ESL (equivalent series inductance) is also an important factor for high frequency converters. Using small,
surface mount ceramic capacitors, placed as close as
possible to the VOUT pins, will minimize ESL.
Low ESR/ESL capacitors should be used to minimize
output voltage ripple. For surface mount applications, AVX
TPS Series tantalum capacitors, Sanyo POSCAP, or Taiyo
Yuden X5R type ceramic capacitors are recommended.
For through-hole applications, Sanyo OS-CON capacitors
offer low ESR in a small package size.
In all applications, a minimum of 4.7μF (generally 22μF is
recommended), low ESR ceramic capacitor should be
placed as close to the VOUT pin as possible, and grounded
to a local ground plane.
3458lfa
11
LTC3458L
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APPLICATIO S I FOR ATIO
Input Capacitor Selection
Compensating the Feedback Loop
The input filter capacitor reduces peak currents drawn
from the input source and reduces input switching noise.
In most applications >1μF per amp of peak input current
is recommended. See Table 3 for a list of capacitor
manufacturers for input and output capacitor selection.
The LTC3458L uses current mode control, with internal
adaptive slope compensation. Current mode control eliminates the 2nd order filter due to the inductor and output
capacitor exhibited in voltage mode controllers, and simplifies the power loop to a single pole filter response. The
product of the modulator control to output DC gain, and
the error amp open-loop gain gives the DC gain of the
system:
Table 3. Capacitor Vendor Information
Supplier
Phone
Website
AVX
(803) 448 - 9411
www.avxcorp.com
Sanyo
(619) 661 - 6322
www.sanyovideo.com
TDK
(847) 803 - 6100
www.component.tdk.com
Murata
Taiyo Yuden
USA: (814) 237-1431
(800) 831-9172
(408) 573 - 4150
www.murata.com
www.t-yuden.com
Operating Frequency Selection
There are several considerations in selecting the operating
frequency of the converter. The first is staying clear of
sensitive frequency bands, which cannot tolerate any
spectral noise. For example in products incorporating RF
communications the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz and in
that case a 1.5MHz switching converter frequency may be
employed. The second consideration is the physical size of
the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The trade off is in efficiency, since the switching losses due
to gate charge increase proportional with frequency.
Thermal Considerations
For the LTC3458L to deliver its full output power, it is
imperative that a good thermal path be provided to dissipate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal
pad on the underside of the IC. It is recommended that
multiple vias in the printed circuit board be used to
conduct heat away from the IC and into a copper plane with
as much area as possible. If the junction temperature rises
above ~150°C, the part will go into thermal shutdown, and
all switching will stop until the temperature drops.
G DC = G CONTROL • G EA •
G CONTROL =
VREF
• G CURRENT _ SENSE
VOUT
2 • VIN
,
IOUT
G EA ≈ 1000
,
G CURRENT _ SENSE =
1
RDS(ON)
The output filter pole is given by:
IOUT
,
π • VOUT • C OUT
where COUT is the output filter capacitor.
fFILTER _ POLE =
The output filter zero is given by:
1
fFILTER _ ZERO =
where RESR
resistance.
,
2π • RESR • C OUT
is the output capacitor equivalent series
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP), and is given by:
2
fRHPZ
VIN
=
2π • IOUT • VOUT • L
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
rolled off before the RHP zero frequency.
3458lfa
12
LTC3458L
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APPLICATIO S I FOR ATIO
The typical error amp compensation is shown in Figure 2.
The equations for the loop dynamics are as follows:
+
ERROR
AMP
fPOLE1 ≈
1
2π • 10e 6 • CC1
1
fZERO1 =
2π • RZ • CC1
1
fPOLE2 ≈
2π • RZ • CC 2
which is close to DC
VOUT
1.225V
–
R1
FB
7
R2
COMP
8
CC1
RZ
CC2
3458L F01
Figure 2
3458lfa
13
LTC3458L
U
TYPICAL APPLICATIO S
Lithium-Ion to 5V, 750mA at 750kHz
COILCRAFT
10μH
MSS7341
Li-Ion to 5VOUT
100
SW
VIN
LTC3458L
2.2μF
10pF
FB
SHDN
ON OFF
1M
COMP
SYNC
324k
0.01μF
0.01μF
90
3.1VIN
3.6VIN
85
80
75
22μF
X5R
10pF
BURST
ILIM
84.5k
4.2VIN
95
33k
SS
RT
274k
VOUT
5V
750mA
VOUT
GND/PGND
EFFICIENCY (%)
Li-Ion
3.1V to 4.2V
70
0.1
560pF
133k
1
10
100
LOAD CURRENT (mA)
1000
3458L TA03b
3458L TA03a
Two Cell to 5VOUT, 250mA at 750kHz
COILCRAFT
10μH
MSS7341
Two Alkaline to 5VOUT
100
SW
VIN
95
LTC3458L
2.2μF
GND/PGND
VOUT
10pF
ON OFF
SHDN
FB
SYNC
COMP
33k
SS
0.01μF
ILIM
274k
84.5k
1M
2.5VIN
85
1.8VIN
80
75
22μF
X5R
10pF
BURST
133k
3.3VIN
90
324k
0.01μF
RT
VOUT
5V
250mA
EFFICIENCY (%)
2 ALKALINE
1.8V to 3.3V
560pF
70
0.1
1
10
100
LOAD CURRENT (mA)
1000
3458L TA04b
3458L TA04a
3458lfa
14
LTC3458L
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PACKAGE DESCRIPTIO
DE/UE Package
12-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1695)
0.58 ±0.05
3.40 ±0.05
1.70 ±0.05
2.24 ±0.05 (2 SIDES)
PACKAGE OUTLINE
0.25 ± 0.05
3.30 ±0.05
(2 SIDES)
0.50
BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 ±0.10
(2 SIDES)
7
R = 0.115
TYP
0.38 ± 0.10
12
R = 0.20
TYP
3.00 ±0.10
(2 SIDES)
1.70 ± 0.10
(2 SIDES)
PIN 1
TOP MARK
PIN 1
NOTCH
(UE12/DE12) DFN 0802
0.200 REF
0.75 ±0.05
0.00 – 0.05
6
0.25 ± 0.05
3.30 ±0.10
(2 SIDES)
1
0.50
BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE A VARIATION OF VERSION
(WGED) IN JEDEC PACKAGE OUTLINE M0-229
2. ALL DIMENSIONS ARE IN MILLIMETERS
3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
4. EXPOSED PAD SHALL BE SOLDER PLATED
3458lfa
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.
15
LTC3458L
U
TYPICAL APPLICATIO
High Current LED Driver with BURST Pin Current Regulation
L1
Two Alkaline Cells Powering
a 1-Watt Lumiled
+
2 ALKALINE
1.8V TO 3.3V
–
SW
VIN
CIN
2.2μF
fOSC = 750kHz
LTC3458L
GND/PGND
100
VOUT
3V TO 3.5V
VOUT
50mA, 3V
90
SHDN
FB
SYNC
COMP
D1
0.01μF
RT
33k
SS
COUT
2.2μF
EFFICIENCY (%)
Z1
ON OFF
150mA, 3.2V
80
300mA, 3.5V
70
0.01μF
ILIM
274k
60
BURST
84.5k
RBURST
NOTE: LUMILED CURRENT REGULATION
~20% OVER VIN RANGE
0.01μF
50
CIN, COUT: TAIYO YUDEN JMK107BJ225MA
D1: LUXEON EMITTER LUMILED WHITE
LXHLMW1D
L1: Coilcraft 10μH MSS7341
RBURST: 45.3k FOR 300mA,
3458L TA06a
82.5k FOR 150mA,
232k FOR 50mA
Z1: CENTRAL SEMI 6.8V ZENER DIODE SOT-23 CMPZ5235B
1.8
3.3
2.3
2.8
INPUT VOLTAGE (V)
3458L TA06b
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3458lfa
16
Linear Technology Corporation
LT 0807 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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