LINER LTC3533EDE

LTC3533
2A Wide Input Voltage
Synchronous Buck-Boost
DC/DC Converter
DESCRIPTION
FEATURES
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Regulated Output with Input Voltages Above,
Below or Equal to the Output
1.8V to 5.5V (Input) and 5.25V (Output)
Voltage Range
0.8A Continuous Output Current: VIN > 1.8V
2A Continuous Output Current: VIN > 3V
Single Inductor
Synchronous Rectification: Up to 96% Efficiency
Programmable Burst Mode® Operation: IQ = 40µA
Output Disconnect in Shutdown
Programmable Frequency from 300kHz to 2MHz
<1µA Shutdown Current
Small Thermally Enhanced 14-Lead (3mm × 4mm ×
0.75mm) DFN package
The LTC®3533 is a wide VIN range, highly efficient, fixed
frequency, buck-boost DC/DC converter that operates
from input voltages above, below or equal to the output
voltage. The topology incorporated in the IC provides a
continuous transfer function through all operating modes,
making the product ideal for single cell lithium-ion/polymer
or multi-cell alkaline/NiMH applications where the output
voltage is within the input voltage range.
The LTC3533 features programmable Burst Mode operation, extended VIN and VOUT ranges down to 1.8V, and
increased output current. Switching frequencies up to
2MHz are programmed with an external resistor. The Burst
Mode threshold is programmed with a single resistor from
the BURST pin to GND.
APPLICATIONS
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Other features include 1µA shutdown current, short
circuit protection, programmable soft-start, current limit
and thermal shutdown. The LTC3533 is housed in the
thermally enhanced 14-lead (3mm × 4mm × 0.75mm)
DFN package.
GSM Modems
Handheld Instruments
Digital Cameras
Smart Phones
Media Players
Miniature Hard Disk Drive Power
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
2.2µH
Efficiency
100
SW2
PVIN
PVOUT
VIN
VOUT
340k
47pF
LTC3533
OFF ON
RUN/SS
FB
107k
RT
10µF
6.49k
VOUT
3.3V
1.5A
330pF
VC
100µF
4.7pF
90
80
EFFICIENCY (%)
VIN
2.4V TO 4.2V
SW1
70
60
50
Burst Mode
OPERATION
VIN = 2.9V
VIN = 2.2V
40
VIN = 3.9V
30
20
BURST
33.2k
SGND
PGND
10
0.1µF
200k
200k
0
0.1
3533 TA01
1
10
100
1000
OUTPUT CURRENT (mA)
10000
3533 TA01b
3533f
1
LTC3533
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
TOP VIEW
VIN, PVIN Voltages ...........................................–0.3 to 6V
VOUT, PVOUT Voltages ......................................–0.3 to 6V
SW1, SW2 Voltages
DC...............................................................–0.3 to 6V
Pulsed < 100ns ...........................................–0.3 to 7V
VC, FB, RUN/SS, BURST Voltages ..................–0.3 to 6 V
Operating Temperature Range (Note 2) ... –40°C to 85°C
Maximum Junction Temperature (Note 3) ............ 125°C
Storage Temperature Range................... –65°C to 125°C
RT
1
BURST
2
14 VC
13 FB
SGND
3
SW1
4
PGND
5
10 VIN
PGND
6
9 PVOUT
SW2
7
8 VOUT
12 RUN/SS
15
11 PVIN
DE PACKAGE
14-LEAD (4mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 43°C/W, θJC = 4.3°C/W
EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER
DE PART MARKING
LTC3533EDE
3533
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 the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, VOUT = 3.3V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Operating Range
●
1.8
5.5
V
Output Voltage Adjust Range
●
1.8
5.25
V
●
1.196
1.22
1.244
V
Feedback Input Current
VFB = 1.22V
1
50
nA
Quiescent Current – Burst Mode Operation
VC = 0V, VBURST = 0V (Note 4)
40
50
µA
Quiescent Current – Shutdown
VRUN = 0V, Not Including Switch Leakage
0.1
1
µA
Quiescent Current – Active
VC = 0V, BURST = 3.6V (Note 4)
700
1100
µA
Feedback Voltage
●
Input Current Limit
3.5
Peak Current Limit
Reverse Current Limit
4.5
A
7
A
0.5
A
NMOS Switch Leakage
Switches B and C
0.1
5
µA
PMOS Switch Leakage
Switches A and D
0.1
10
µA
NMOS Switch On Resistance
Switches B and C
60
mΩ
PMOS Switch On Resistance
Switches A and D
80
mΩ
Maximum Duty Cycle
Boost (% Switch C On)
Buck (% Switch A On)
90
%
%
Minimum Duty Cycle
●
●
●
80
100
0
%
3533f
2
LTC3533
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, VOUT = 3.3V, unless otherwise noted.
PARAMETER
CONDITIONS
Frequency Accuracy
RT = 33.2k
●
MIN
TYP
MAX
0.7
1
1.3
UNITS
MHz
Error Amp AVOL
80
dB
Error Amp Source Current
–20
µA
Error Amp Sink Current
250
µA
1
V
Burst Threshold
Burst Input Current
VBURST = 5.5V, VIN = 5.5V
RUN/SS Threshold
When IC is Enabled
When EA is at Maximum Boost Duty Cycle
RUN/SS Input Current
VRUN = 5.5V
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.
Note2: The LTC3533EDE is guaranteed to meet performance specifications
from 0ºC to 85ºC. Specifications over the –40ºC to 85ºC operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
●
0.4
8
µA
0.7
1.3
1.4
V
V
0.01
1
µA
Note 3: This IC includes over-temperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when over-temperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may result in device degradation or failure.
Note 4: Current Measurements are performed when the outputs are not
switching.
3533f
3
LTC3533
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current vs VIN
(Fixed Frequency Mode)
3.0
1.5 MHz
2.5
1.0 MHz
1.5
0.5 MHz
1.0
NO SWITCHING
50
5
45
4
40
3
CURRENT LIMIT CHANGE (%)
VIN QUIESCENT CURRENT (µA)
VIN QUIESCENT CURRENT (mA)
2.0 MHz
2.0
35
30
25
20
15
10
2
1
0
–1
–2
–3
0.5
5
–4
0.0
2.5
0
–5
–55 –35 –15
3.0
3.5
4.0
VIN (V)
4.5
5.0
2.5
5.5
3.0
3.5
4.0
VIN (V)
4.5
5.0
5.5
3533 G03
Minimum Start Voltage vs
Temperature
Automatic Burst Mode Threshold
vs RBURST
Average Input Current Limit vs
Temperature
1.84
LEAVE Burst Mode
OPERATION
100
ENTER Burst Mode
OPERATION
50
5
4
1.82
3
CHANGE FROM 25°C (%)
MINIMUM START VOLTAGE (V)
200
150
5 25 45 65 85 105 125
TEMPERATURE (°C)
3530 G02
3533 G01
LOAD CURRENT (mA)
Peak Current Limit vs
Temperature
Burst Mode Quiescent Current
4.0
3.5
TA = 25°C, unless otherwise specified.
1.80
1.78
1.76
1.74
2
1
0
–1
–2
–3
1.72
–4
0
100
125
150
175
200
RBURST (kΩ)
225
1.70
–45 –25 –5
250
3533 G04
15
35
55
75
TEMPERATURE (°C)
95 115
–5
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3533 G05
Frequency Change vs
Temperature
3533 G06
Switch Pins Before Entering
Boost Mode
Feedback Voltage vs Temperature
1.076
1.2250
1.074
FEEDBACK VOLTAGE (V)
FREQUENCY (MHz)
1.072
1.070
1.068
1.066
1.064
1.062
1.2200
SW1
2V/DIV
1.2150
SW2
2V/DIV
1.060
1.058
1.056
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3533 G07
1.2100
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3533 G08
50ns/DIV
3533 G09
VIN = 2.9V
VOUT = 3.3V AT 500mA
3533f
4
LTC3533
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise specified.
Switch Pins Entering Buck-Boost
Mode
Switch Pins in Buck-Boost Mode
SW1
2V/DIV
SW1
2V/DIV
Output Ripple at 1A Load
VIN = 2.7V
VIN = 3.3V
SW2
2V/DIV
SW2
2V/DIV
50ns/DIV
VIN = 3.3V
VOUT = 3.3V AT 500mA
VIN = 4.2V
3533 G10
50ns/DIV
3533 G11
1µs/DIV
VOUT = 3.3V, 20mV/DIV
COUT = 100µF CERAMIC
3533 G12
VIN = 4.2V
VOUT = 3.3V AT 500mA
Load Transient Response in Fixed
Frequency Mode, No Load to 1.5A
Load Transient Response in Auto
Burst Mode, No Load to 600mA
VOUT
100mV/DIV
VOUT
100mV/DIV
IL
0.5A/DIV
LOAD
0.5A/DIV
100µs/DIV
VIN = 3.6V
VOUT = 3.3V
COUT = 100µF X5R CERAMIC
100µs/DIV
VIN = 3.6V
VOUT = 3.3V
COUT = 100µF X5R CERAMIC +
100µF LOW ESR TANTALUM
3533 G13
Transition from Burst Mode
Operation to Fixed Frequency Mode
Burst Mode Operation
VOUT
50mV/DIV
VOUT
50mV/DIV
INDUCTOR
CURRENT
0.5A/DIV
INDUCTOR
CURRENT
0.5A/DIV
20µs/DIV
COUT = 100µF CERAMIC
3533 G14
3533 G15
200µs/DIV
COUT = 100µF CERAMIC
3533 G16
3533f
5
LTC3533
PIN FUNCTIONS
RT (Pin 1): Programs the Frequency of the Internal Oscillator. Connect a resistor from RT to ground.
f(kHz) = 33,170/RT (kΩ)
BURST (Pin 2): Used to set the Automatic Burst Mode
Threshold. Connect a resistor and capacitor in parallel
from this pin to ground. See the Applications Information
section for component value selection. For manual control,
ground the pin to force Burst Mode operation, connect to
VIN to force fixed frequency PWM mode.
SGND (Pin 3): Signal ground for the IC.
SW1 (Pin 4): Switch Pin where Internal Switches A and B
are Connected. Connect inductor from SW1 to SW2. An
optional Schottky diode can be connected from SW1 to
ground for a moderate efficiency improvement. Minimize
trace length to reduce EMI.
PGND1, PGND2 (Pins 5, 6): Power Ground for the Internal
NMOS Power Switches.
SW2 (Pin 7): Switch Pin where Internal Switches C and
D are Connected. An optional Schottky diode can be
connected from SW2 to VOUT for a moderate efficiency
improvement. For applications with output voltages over
4.3V, this Schottky diode is required to ensure the SW2
pin does not exhibit excess voltage. Minimize trace length
to reduce EMI.
VOUT (Pin 8): Voltage Sensing Pin for PVOUT and Input
Supply Pin for Internal Circuitry Powered by PVOUT. A filter
capacitor is placed from VOUT to GND. A ceramic bypass
capacitor is recommended as close to the VOUT and GND
pins as possible.
PVOUT (Pin 9): Output of the Synchronous Rectifier. A
filter capacitor is placed from PVOUT to PGND. A ceramic
bypass capacitor is recommended as close to the PVOUT
and PGND pins as possible.
VIN (Pin 10): Input Supply Pin. Internal VCC for the IC.
PVIN (Pin 11): Power VIN Supply Pin. A 10µF ceramic
capacitor is recommended as close to the PVIN and PGND
pins as possible.
RUN/SS (Pin 12): Combined Enable and Soft-Start. Applied
voltage <0.4V shuts down the IC. Tie to >1.4V to enable
the IC and >1.6V to ensure the error amp is not clamped
from soft-start. An RC from the shutdown command signal
to this pin will provide a soft-start function by limiting the
rise time of VC
FB (Pin 13): Feedback Pin. Connect resistor divider tap
here. The output voltage can be adjusted from 1.8V to
5.25V. The feedback reference voltage is typically 1.22V.
VOUT = 1.22 •
R1+ R2
R2
VC (Pin 14): Error Amp Output. An R-C network is connected from this pin to FB for loop compensation. Refer
to “Closing the Feedback Loop” section for component
selection guidelines. During Burst Mode operation, VC is
internally connected to a hold circuit.
Exposed Pad (Pin 15): IC Substrate Ground. This pin must
be soldered to the PCB ground to provide both electrical
contact and a good thermal contact to the PCB.
3533f
6
LTC3533
BLOCK DIAGRAM
SW2
SW1
VOUT
SW D
SW B
GATE
DRIVERS
AND
ANTI-CROSS
CONDUCTION
–0.5A
SW C
–
SW A
+
VIN
1.8V TO 5.5V
ISENSE
AMP
REVERSE
CURRENT
LIMIT
R1
+
+
SUPPLY
CURRENT
LIMIT
+
ERROR
AMP
+
–
UVLO
+
RT
PWM
LOGIC
AND
OUTPUT
PHASING
CLAMP
PWM
COMPARATORS
VC
+
–
–
1.6V
FB
–
–
4.5A
1.22V
RT
R2
OSC
SLEEP
BURST MODE
OPERATION
CONTROL
BURST
RUN/SS
RUN
0 = BURST MODE
1 = FIXED FREQUENCY
RSS
VIN
GND
CSS
3533 BD
OPERATION
The LTC3533 provides high efficiency, low noise power
for a wide variety of handheld electronic devices. The LTC
proprietary topology allows input voltages above, below
or equal to the output voltage by properly phasing the
output switches. The error amplifier output voltage on VC
determines the output duty cycle of the switches. Since
VC is a filtered signal, it provides rejection of frequencies
well below the switching frequency. The low RDS(ON), low
gate charge synchronous switches provide high frequency
pulse width modulation control at high efficiency. High
efficiency is achieved at light loads when Burst Mode
operation is entered and the LTC3533’s quiescent current
drops to a low 40µA.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is programmed by an external
resistor from RT to ground, according to the following
equation:
f(kHz) = 33,170/RT(k)
3533f
7
LTC3533
OPERATION
Error Amplifier
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier (from FB to VC) to obtain stability of the converter.
For improved bandwidth, an additional RC feed-forward
network can be placed across the upper feedback divider
resistor. The voltage on the RUN/SS pin clamps the error
amplifier output, VC, to provide a soft-start function.
Supply Current Limits
There are two different supply current limit circuits in the
LTC3533, working consecutively, each having internally
fixed thresholds which vary inversely with VIN.
The first circuit is a current limit amplifier, sourcing current into FB to drop the output voltage, should the peak
input current exceed 4.5A typical. This method provides a
closed loop means of clamping the input current. During
conditions where VOUT is near ground, such as during a
short circuit or startup, this threshold is cut to 750mA,
providing a fold-back feature. For this current limit feature
to be most effective, the Thevenin resistance from FB to
ground should be greater than 100k.
Should the peak input current exceed 7A typical, the second
circuit, a high speed peak current limit comparator, shuts
off PMOS switch A. The delay to output of this comparator
is typically 50ns.
Reverse Current Limit
During fixed frequency operation, the LTC3533 operates in
forced continuous conduction mode. The reverse current
limit comparator monitors the inductor current from the
output through switch D. Should this negative inductor
current exceed 500mA typical, the LTC3533 shuts off
switch D.
Four-Switch Control
Figure 1 shows a simplified diagram of how the four internal switches are connected to the inductor, VIN, VOUT
and GND.Figure 2 shows the regions of operation for the
LTC3533 as a function of the control voltage, VC.
Dependent on VC’s magnitude, the LTC3533 will operate
in either buck, buck/boost or boost mode. The four power
switches are properly phased so the transfer between operating modes is continuous, smooth and transparent to
the user. When VIN approaches VOUT the buck/boost region
is entered, where the conduction time of the four switch
region is typically 150ns. Referring to Figures 1 and 2, the
various regions of operation will now be described.
Buck Region (VIN > VOUT)
Switch D is always on and switch C is always off during
this mode. When the control voltage, VC, is above voltage V1, switch A begins to switch. During the off time of
switch A, synchronous switch B turns on for the remainder
of the period. Switches A and B will alternate similar to a
typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until
the maximum duty cycle of the converter in buck mode
reaches DMAX_BUCK, given by:
DMAX_BUCK = 100 – D4SW %
where D4SW = duty cycle % of the four switch range.
85%
DMAX
BOOST
PVIN
PVOUT
11
9
SW1
3
NMOS B
A ON, B OFF
BOOST REGION
PWM CD SWITCHES
DMIN
BOOST
PMOS D
PMOS A
L1
V4 (≈1.5V)
DMAX
BUCK
SW2
V3 (≈1.15V)
FOUR SWITCH PWM
BUCK/BOOST REGION
V2 (≈1V)
D ON, C OFF
PWM AB SWITCHES BUCK REGION
7
NMOS C
3533 F01
Figure 1. Simplified Diagram of Output Switches
V1 (≈0.7V)
0%
DUTY
CYCLE
3533 F02
CONTROL
VOLTAGE, VC
Figure 2. Switch Control vs Control Voltage, VC
3533f
8
LTC3533
OPERATION
D4SW = (150ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the “four switch,” or buck/boost region
is reached.
Buck/Boost or Four Switch (VIN ~ VOUT)
When the control voltage, VC, is above voltage V2, switch
pair AD remain on for duty cycle DMAX_BUCK, and switch
pair AC begins to phase in. As switch pair AC phases in,
switch pair BD phases out accordingly. When VC reaches
the edge of the buck/boost range, at voltage V3, the AC
switch pair completely phase out the BD pair, and the boost
phase begins at duty cycle D4SW. The input voltage, VIN,
where the four switch region begins is given by:
VIN = VOUT(1 – D) = VOUT(1 – 150ns • f) V
The point at which the four switch region ends is given
by:
VIN =
VOUT
V
1− (150ns • f)
where f = operating frequency, Hz.
Boost Region (VIN < VOUT)
Switch A is always on and switch B is always off during
this mode. When the control voltage, VC, is above voltage V3, switch pair CD will alternately switch to provide
a boosted output voltage. This operation is typical to a
synchronous boost regulator. The maximum duty cycle
of the converter is limited to 90% typical and is reached
when VC is above V4.
BURST MODE OPERATION
Burst Mode operation reduces the LTC3533’s quiescent
current consumption at light loads and improves overall
conversion efficiency, increasing battery life. During Burst
Mode operation the LTC3533 delivers energy to the output until it is regulated and then goes into a sleep mode
where the outputs are off and the quiescent current drops
to 40µA. In this mode the output ripple has a variable
frequency component that depends upon load current,
and will typically be about 2% peak-to-peak. Burst Mode
operation ripple can be reduced slightly by using more
output capacitance. Another method of reducing Burst
Mode operation ripple is to place a small feed-forward
capacitor across the upper resistor in the VOUT feedback
divider network (as in Type III compensation).
During the period where the device is delivering energy
to the output, the peak switch current will rise to 450mA
typical and the inductor current will terminate at zero current for each cycle. In this mode, the typical maximum
average output currents are given by:
IMAX(BURST)BUCK ≈ 225mA; VOUT < VIN
IMAX(BURST)BOOST ≈ 225mA • (VIN/VOUT); VOUT > VIN
IMAX(BURST)BUCK-BOOST ≈ 350mA; VOUT ≈ VIN, since the
input and output are connected together for most of the
cycle.
The efficiency below 1mA becomes dominated primarily
by the quiescent current. The Burst Mode operation efficiency is given by:
Efficiency ≅
η • ILOAD
40µA + ILOAD
where η is typically 90% during Burst Mode operation
Programmable Automatic Burst Mode Operation
Burst Mode operation can be automatic or digitally controlled with a single pin. In automatic mode, the LTC3533
enters Burst Mode operation at the programmed threshold
and returns to fixed frequency operation when the load
demand increases. The load current at which the mode
transition occurs is programmed using a single external
resistor from BURST to ground, according to the following equations:
Enter Burst Mode Operation : IBURST =
Exit Burst Mode Operation : IBURST =
17
RBURST
19
RBURST
Where RBURST is in kΩ and IBURST is the load transition
current in Amps. Do not use values of RBURST greater
than 1MΩ.
3533f
9
LTC3533
OPERATION
For automatic operation a filter capacitor must also be
connected from BURST to ground. The equation for the
minimum capacitor value is:
CBURST(MIN) ≥
COUT • VOUT
60, 000
where CBURST(MIN) and COUT are in µF.
In the event that a load transient causes FB to drop by
more than 4% from the regulation value while in Burst
Mode operation, the LTC3533 will immediately switch
to fixed frequency mode and an internal pull-up will be
momentarily applied to BURST, rapidly charging CBURST.
This prevents the IC from immediately re-entering Burst
mode operation once the output achieves regulation.
Manual Burst Mode Operation
For manual control of Burst Mode operation, the RC
network connected to BURST can be eliminated. To force
fixed frequency mode, BURST should be connected to
VIN. To force Burst Mode operation, BURST should be
grounded. When commanding Burst Mode operation
manually, the circuit connected to BURST should be able
to sink up to 2mA.
For optimum transient response with large dynamic loads,
the operating mode should be controlled digitally by the
host. By commanding fixed frequency operation prior to a
sudden increase in load, output voltage droop can be minimized. Note that if the load current applied during forced
Burst Mode operation (BURST pin is grounded) exceeds
the current that can be supplied, the output voltage will
start to droop and the LTC3533 will automatically come out
of Burst Mode operation and enter fixed frequency mode,
raising VOUT. Once regulation is achieved, the LTC3533 will
then enter Burst Mode operation once again (since the user
is still commanding this by grounding BURST), and the
cycle will repeat, resulting in about 4% output ripple.
Burst Mode Operation to Fixed Frequency Transient
Response
In Burst Mode operation, the compensation network is
not used and VC is disconnected from the error amplifier.
During long periods of Burst Mode operation, leakage
currents in the external components or on the PC board
could cause the compensation capacitor to charge (or
discharge), which could result in a large output transient
when returning to fixed frequency mode of operation, even
at the same load current. To prevent this, the LTC3533
incorporates an active clamp circuit that holds the voltage
on VC at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
fixed frequency mode operation. For optimum transient
response, Type 3 compensation is also recommended
to broad band the control loop and roll off past the two
pole response of the output LC filter. (See Closing the
Feedback Loop).
Soft-Start
The soft-start function is combined with shutdown. When
the RUN/SS pin is brought above 1V typical, the LTC3533
is enabled but the error amplifier duty cycle is clamped
from VC. A detailed diagram of this function is shown in
Figure 3. The components RSS and CSS provide a slow
ramping voltage on RUN/SS to provide a soft-start function.
To ensure that VC is not being clamped, RUN/SS must be
raised above 1.6V.
VIN
RUN/SS
VC
3533 F03
Figure 3.
3533f
10
LTC3533
APPLICATIONS INFORMATION
COMPONENT SELECTION
where f = switching frequency, Hz
∆IL = maximum allowable inductor ripple current
1
RT
VC
14
2
BURST
FB
13
VIN(MAX) = maximum input voltage
3
SGND
RUN/SS
12
VOUT = output voltage
4
SW1
PVIN
11
5
PGND
VIN
10
6
PGND
PVOUT
9
VIN(MIN) = minimum input voltage
VIN
VOUT
7
SW2
8
VOUT
GND
MULTIPLE VIAS
3533 F04
Figure 4. Recommended Component Placement. Traces Carrying
High Current Should be Short and Wide. Trace Area at FB and VC
Pins are Kept Low. Lead Length to Battery Should be Kept Short.
PVOUT and PVIN Ceramic Capacitors Close to the IC Pins.
Inductor Selection
The high frequency operation of the LTC3533 allows the
use of small surface mount inductors. The inductor ripple
current is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
LBOOST >
LBUCK >
VIN(MIN)2 • ( VOUT − VIN(MIN) )
f • ∆IL • VOUT2
VOUT • ( VIN(MAX ) − VOUT )
f • ∆IL • VIN(MAX )
For high efficiency, choose a ferrite inductor with a high
frequency core material 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 the
peak inductor currents in the 4A to 6A region. To minimize
radiated noise, use a shielded inductor. See Table 1 for a
suggested list of inductor suppliers.
Output Capacitor Selection
The bulk value of the output filter capacitor is set to reduce
the ripple due to charge into the capacitor each cycle. The
steady state ripple due to charge is given by:
%Ripple _ Boost =
%Ripple _ Buck =
IOUT(MAX ) •( VOUT − VIN(MIN) ) • 100
COUT • VOUT2 • f
( VIN(MAX ) − VOUT ) • 100
8L COUT • VIN(MAX ) • f 2
%
%
where COUT = output filter capacitor
H
IOUT(MAX) = maximum output load current
The output capacitance is usually many times larger than
the minimum value in order to handle the transient response
H
Table 1. Inductor Vendor Information
SUPPLIER
PHONE
FAX
WEB SITE
Coilcraft
(847) 639-6400
(847) 639-1469
www.coilcraft.com
CoEv Magnetics
(800) 227-7040
(650) 361-2508
www.circuitprotection.com/magnetics.asp
Murata
(814) 237-1431
(800) 831-9172
(814) 238-0409
www.murata.com
Sumida
USA: (847) 956-0666
Japan: 81(3) 3607-5111
USA: (847) 956-0702
Japan: 81(3) 3607-5144
www.sumida.com
TDK
(847) 803-6100
(847) 803-6296
www.component.tdk.com
TOKO
(847) 297-0070
(847) 699-7864
www.tokoam.com
3533f
11
LTC3533
APPLICATIONS INFORMATION
requirements of the converter. As a rule of thumb, the ratio
of the operating frequency to the unity-gain bandwidth of
the converter is the amount the output capacitance will
have to increase from the above calculations in order to
maintain the desired transient response.
to provide the conduction path to the output. Note that
Burst Mode operation is inhibited at output voltages below
1V typical.
The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden or
TDK ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. See Table 2 for
contact information.
A Schottky diode from SW2 to VOUT is required for output
voltages over 4.3V. The diode must be located as close to
the pins as possible in order to reduce the peak voltage on
SW2 due to parasitic lead and trace inductances.
Output Voltage > 4.3V
Input Voltage > 4.5V
Since PVIN is the supply voltage for the IC it is recommended to place at least a 4.7µF, low ESR ceramic bypass
capacitor close to PVIN and GND. It is also important to
minimize any stray resistance from the converter to the
battery or other power source.
For applications with input voltages above 4.5V which
could exhibit an overload or short-circuit condition, a
2Ω/1nF series snubber is required between SW1 and
GND. A Schottky diode from SW1 to PVIN should also be
added as close to the pins as possible. For the higher input
voltages, VIN bypassing becomes more critical. Therefore,
a ceramic bypass capacitor as close to the PVIN and GND
pins as possible is also required.
Optional Schottky Diodes
Operating Frequency Selection
Schottky diodes across the synchronous switches B and
D are not required, but do provide a lower drop during the
break-before-make time (typically 15ns), thus improving
efficiency. Use a surface mount Schottky diode such as an
MBRM120T3 or equivalent. Do not use ordinary rectifier
diodes since their slow recovery times will compromise
efficiency.
Higher operating frequencies allow the use of a smaller
inductor and smaller input and output filter capacitors,
thus reducing board area and component height. However, higher operating frequencies also increase the IC’s
total quiescent current due to the gate charge of the four
switches, as given by:
Buck:
IQ = (600e – 12 • VIN • f ) mA
Output Voltage < 1.8V
Boost:
IQ = [800e – 12 • (VIN + VOUT) • f ] mA
The LTC3533 can operate as a buck converter with output
voltages as low as 400mV. The part is specified at 1.8V
minimum to allow operation without the requirement of a
Schottky diode; Since synchronous switch D is powered
from PVOUT, and the RDS(ON) will increase at low output
voltages, a Schottky diode is required from SW2 to VOUT
Buck/Boost: IQ = [(1400e – 12 • VIN + 400e – 12 •
VOUT) • f ] mA
Input Capacitor Selection
where f = switching frequency in Hz. Therefore frequency
selection is a compromise between the optimal efficiency
and the smallest solution size.
Table 2. Capacitor Vendor Information
SUPPLIER
PHONE
FAX
WEB SITE
AVX
(803) 448-9411
(803) 448-1943
www.avxcorp.com
Sanyo
(619) 661-6322
(619) 661-1055
www.sanyovideo.com
Taiyo Yuden
(408) 573-4150
(408) 573-4159
www.t-yuden.com
TDK
(847) 803-6100
(847) 803-6296
www.component.tdk.com
3533f
12
LTC3533
APPLICATIONS INFORMATION
Closing the Feedback Loop
The LTC3533 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(buck, boost, buck/boost), but is usually no greater than
15. The output filter exhibits a double pole response, as
given by:
f FILTER _ POLE =
1
Hz
2 • π • L • COUT
f UG =
(in buck mod e)
f FILTER _ POLE =
A simple Type I compensation network can be incorporated
to stabilize the loop, but at a cost of reduced bandwidth
and slower transient response. To ensure proper phase
margin using Type I compensation, the loop must be
crossed over a decade before the LC double pole. Referring
to Figure 5, the unity-gain frequency of the error amplifier
with the Type I compensation is given by:
VIN
Hz
2 • VOUT • π • L • COUT
(in boost mod e)
where L is in Henries and COUT is in Farads.
1
Hz
2 • π • R1• CP1
Most applications demand an improved transient response
to allow a smaller output filter capacitor. To achieve a higher
bandwidth, Type III compensation is required, providing
two zeros to compensate for the double-pole response of
the output filter. Referring to Figure 6, the location of the
poles and zeros are given by:
The output filter zero is given by:
f FILTER _ ZERO =
1
2 • π • RESR • COUT
Hz
2 • π • 10e3 • R1• CP1
(which is a very low frequency)
Hz
where RESR is the equivalent series resistance of the
output capacitor.
A troublesome feature in boost mode is the right-half plane
zero (RHP), given by:
f RHPZ =
1
f POLE1 =
VIN2
Hz
2 • π • IOUT • L • VOUT
The loop gain is typically rolled off before the RHP zero
frequency.
1
Hz
2 • π • RZ • CP1
1
f ZERO2 =
Hz
2 • π • R1• CZ1
1
f POLE2 =
Hz
2 • π • RZ • CP2
f ZERO1 =
where resistance is in Ohms and capacitance is in Farads.
VOUT
+
VOUT
+
ERROR
AMP
–
1.22V
R1
FB
11
–
1.22V
CP1
R2
3533 F05
Figure 5. Error Amplifier with Type I Compensation
R1
CZ1
FB
12
VC
12
VC
ERROR
AMP
CP1
RZ
R2
11
CP2
3533 F06
Figure 6. Error Amplifier with Type III Compensation
3533f
13
LTC3533
TYPICAL APPLICATIONS
High Efficiency, High Current LED Driver
3.3µH
11
VIN
3V TO 4.2V
10
4
7
SW1
SW2
PVIN
PVOUT
VIN
VOUT
9
ILED = 1A
8
4.7µF
LTC3533
OFF ON
12
RUN/SS
FB
13
1nF
1
10µF
RT
VC
BURST
14
SGND PGND
44.2k
3
5
100k
2
47pF
95.3k
6
100k
301k
3533 TA02
1MHz Li-Ion to 3.6V at 2A, Pulsed, with Manual Mode Control
6.8µH
11
VIN
3V TO 4.2V
10
4
7
SW1
SW2
PVIN
PVOUT
VIN
VOUT
9
VOUT
3.6V AT 2A
8
388k
220pF
LTC3533
OFF ON
12
1
10µF
64.9k
RUN/SS
FB
RT
VC
13
14
15k
470pF
2
BURST
FIXED
BURST
FREQUENCY
SGND PGND
3
5
2.2k
6
200µF
200k
3533 TA03
3533f
14
LTC3533
PACKAGE DESCRIPTION
DE Package
14-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1708 Rev A)
0.70 ±0.05
3.60 ±0.05
1.70 ±0.05
2.20 ±0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50
BSC
3.30 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
R = 0.115
TYP
4.00 ±0.10
(2 SIDES)
R = 0.05
TYP
3.00 ±0.10
(2 SIDES)
8
0.40 ± 0.10
14
1.70 ± 0.05
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
PIN 1
TOP MARK
(SEE NOTE 6)
(DE14) DFN 0905 REV A
7
0.200 REF
1
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
0.00 – 0.05
3.30 ±0.05
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WGED-3) IN JEDEC
PACKAGE OUTLINE MO-229
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. 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
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3533f
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
LTC3533
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC3400/LT3400B
600mA (ISW), 1.2MHz Synchronous Step-Up DC/DC Converter
VIN: 0.85V to 5V, VOUT(MAX) = 5V,
IQ = 19µA/300µA, ISD < 1µA, ThinSOT Package
LTC3401/LT3402
1A/2A (ISW), 3MHz Synchronous Step-Up DC/DC Converter
VIN: 0.5V to 5V, VOUT(MAX) = 6V, IQ = 38mA,
ISD < 1µA, MS Package
LTC3405/LTC3405A 300mA (IOUT), 1.5MHz Synchronous Step-Up DC/DC Converter
VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 20µA,
ISD ≤ 1µA, MS10 Package
LTC3406/LTC3406B 600mA (IOUT), 1.5MHz Synchronous Step-Up DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20µA,
ISD ≤ 1µA, ThinSOT Package
LTC3407
600mA (IOUT), 1.5MHz Dual Synchronous Step-Up DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40µA,
ISD ≤ 1µA, MS Package
LTC3411
1.25A (IOUT), 4MHz Synchronous Step-Up DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60µA,
ISD ≤ 1µA, MS Package
LTC3412
2.5A (IOUT), 4MHz Synchronous Step-Up DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60µA,
ISD ≤ 1µA, TSSOP16E Package
LTC3421
3A (ISW), 3MHz Synchronous Step-Up DC/DC Converter
VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12µA,
ISD < 1µA, QFN Package
LTC3425
5A (ISW), 8MHz Multiphase Synchronous Step-Up DC/DC Converter
VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12µA,
ISD < 1µA, QFN Package
LTC3429
600mA (ISW), 500kHz Synchronous Step-Up DC/DC Converter
VIN: 0.5V to 4.4V, VOUT(MAX: 5V, IQ = 20µA,
ISD < 1µA, ThinSOT Package
LTC3440
600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MAX): 2.5V to 5.5V, IQ = 25µA,
ISD < 1µA, MS, DFN Package
LTC3441
1.2A (IOUT), 1MHz Synchronous Buck-Boost DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MAX): 2.4V to 5.5V, IQ = 25µA,
ISD < 1µA, DFN Package
LTC3442/LTC3443
1.2A (IOUT), Synchronous Buck-Boost DC/DC Converters,
LTC3442 (1MHz), LTC3443 (600kHz)
VIN: 2.4V to 5.5V, VOUT(MAX): 2.4V to 5.25V, IQ = 28µA,
ISD < 1µA, DFN Package
LTC3444
500mA (IOUT), Synchronous Buck-Boost DC/DC Converter
VIN: 2.7V to 5.5V, VOUT = 0.5V to 5V, DFN Package,
Internal Compensation
LTC3530
600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter
VIN: 1.8V to 5.5V, VOUT: 1.8V to 5.25V, IQ = 40µA,
ISD < 1µA, 10-Pin MSOP Package, 3mm × 3mm DFN
LTC3532
500mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter
VIN: 2.4V to 5.5V, VOUT: 2.4V to 5.5V, IQ = 35µA,
ISD < 1µA, 10-Pin MSOP Package, 3mm × 3mm DFN
Thin SOT is a trademark of Linear Technology Corporation.
3533f
16 Linear Technology Corporation
LT 0207 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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