LTC3533 Datasheet, PDF (8/16 Pages) Linear Technology – 2A Wide Input Voltage Synchronous Buck-Boost DC/DC Converter

LTC3533
OPERATION
Error Amplifier
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier (from FB to V_C) 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, V_C, 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 V_IN.
The first circuit is a current limit amplifier, sourcing cur-
rent 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 V_OUT 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
PV_IN
11
PV_OUT
9
PMOS A
SW1 L1
SW2
3
7
NMOS B
PMOS D
NMOS C
3533 F01
Figure 1. Simplified Diagram of Output Switches
8
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 in-
ternal switches are connected to the inductor, V_IN, V_OUT
and GND.Figure 2 shows the regions of operation for the
LTC3533 as a function of the control voltage, V_C.
Dependent on V_C’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 op-
erating modes is continuous, smooth and transparent to
the user. When V_IN approaches V_OUT 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 (V_IN > V_OUT)
Switch D is always on and switch C is always off during
this mode. When the control voltage, V_C, is above volt-
age 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 volt-
age increases, the duty cycle of switch A increases until
the maximum duty cycle of the converter in buck mode
reaches D_{MAX_BUCK}, given by:
D_{MAX_BUCK} = 100 – D4_SW %
where D4_SW = duty cycle % of the four switch range.
85%
D_MAX
BOOST
D_MIN
BOOST
D_MAX
BUCK
A ON, B OFF
PWM CD SWITCHES
BOOST REGION
FOUR SWITCH PWM
BUCK/BOOST REGION
D ON, C OFF
PWM AB SWITCHES
BUCK REGION
V4 (≈1.5V)
V3 (≈1.15V)
V2 (≈1V)
0%
V1 (≈0.7V)
DUTY
CYCLE
CONTROL
3533 F02 _{VOLTAGE, VC}
Figure 2. Switch Control vs Control Voltage, V_C
3533f