LTC3531 Datasheet, PDF (9/16 Pages) Linear Technology – 200mA Buck-Boost Synchronous DC/DC Converters

LTC3531/
LTC3531-3.3/LTC3531-3
OPERATIO
The LTC3531, LTC3531-3.3 and LTC3531-3 synchro-
nous buck-boost converters utilize a Burst Mode control
technique to achieve high efficiency over a wide dynamic
range of load currents. A 2% accurate comparator is
used to monitor the output voltage. If V_OUT is above its
programmed reference threshold no switching occurs and
only quiescent current is drawn from the power source
(sleep mode). When V_OUT drops below the reference
threshold the IC “wakes up”, switching commences, and
the output capacitor is charged. The value of the output
capacitor, the load current, and the comparator hysteresis
(~1%) determines the number of current pulses required
to pump-up the output capacitor before the part returns
to sleep.
In order to determine the best operating mode for the
converter, the LTC3531 contains a second comparator
that monitors the relative voltage difference between V_IN
and V_OUT. Input and output voltages in the various modes
as well as typical inductor currents are shown in Figure 1.
Regions of the current waveforms where switches A and
D are on provide the highest efficiency since energy is
transferred directly from the input source to the output.
Boost Mode
If V_IN is ~400mV below V_OUT, the LTC3531 operates in
boost or step-up mode. Referring to Figure 1 (left side)
when V_OUT falls below its regulation voltage, switches A
and C are turned on (V_IN is applied across the inductor)
and current is ramped until I_PEAK is detected. When this
occurs, C is turned off, D is turned on and current is deliv-
ered to the output capacitor (V_IN – V_OUT is applied across
the inductor). Inductor current falls when D is on, until an
I_VALLEY is detected. Terminating at I_VALLEY, rather than I_ZERO,
results in an increased load current capability for a given
peak current. This AC then AD switch sequence is repeated
until the output is pumped above its regulation voltage, a
final I_ZERO is detected, and the part returns to sleep mode
(I_VALLEY is ignored and I_ZERO is used in all modes once
V_OUT is above its programmed value).
4-Switch Mode
If (V_OUT – 400mV) < ~V_IN < (V_OUT + 800mV), the LTC3531
operates in 4-switch step-up/down mode. Returning to
Figure 1 (center) when V_OUT falls below its regulation volt-
age, switches A and C are turned on and current is ramped
until I_PEAK is detected. As with Boost Mode operation, C
is then turned off, D is turned on and current is delivered
to the output. When A and D are on, the inductor current
slope is dependant on the relationship between V_IN, V_OUT,
and the R_DSON of the switches. In 4-switch mode, a t_OFF
timer (approximately 3μs) is used to terminate the AD
pulse. Once the t_OFF timer expires, switch A is turned off,
B is turned on and inductor current is ramped down (V_OUT
is applied across the inductor) until I_VALLEY is detected.
This sequence is repeated until the output is regulated,
BD switches are turned on, and a final I_ZERO is detected.
Anticross conduction circuitry in all modes ensures the
P-channel MOSFET and N-channel MOSFET switch pairs (A
and B or D and C) are never turned on simultaneously.
V_IN
V_OUT
A
D
L
SW1
SW2
V_OUT
V_IN
I_MAX
B
C
I_PEAK
I_VALLEY
t_OFF
t_OFF
t_OFF
I_ZERO
AC AD AC AD
BOOST MODE
AC AD BD AC AD BD AC AD BD
4SW MODE
Figure 1. Voltage and Current Waveforms
V_IN
AD BD AD BD
BUCK MODE
3531 F01
3531fb
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