Datasheet

LTC3531/
LTC3531-3.3/LTC3531-3
9
3531fa
The LTC3531, LTC3531-3.3 and LTC3531-3 synchro-
nous buck-boost converters utilize a Burst Mode control
technique to achieve high effi ciency 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 effi ciency 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
fi nal 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 fi nal 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.
OPERA TIO
U
Figure 1. Voltage and Current Waveforms
V
OUT
I
MAX
BOOST MODE 4SW MODE BUCK MODE
I
PEAK
I
VALLEY
I
ZERO
V
IN
V
IN
t
OFF
t
OFF
t
OFF
AC AC AC AD AD ADADAD AD AD ACAC BD BDBD BD BD
3531 F01
V
IN
V
OUT
A
BC
D
SW1 SW2
L