LTC3531/
LTC3531-3.3/LTC3531-3
U
OPERATIO
occurs, C is turned off, D is turned on and current is deliv-
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
ered to the output capacitor (V – V
is applied across
IN
OUT
the inductor). Inductor current falls when D is on, until an
isdetected.TerminatingatI ,ratherthanI
I
,
ZERO
VALLEY
VALLEY
results in an increased load current capability for a given
peakcurrent.ThisACthenADswitchsequenceisrepeated
until the output is pumped above its regulation voltage, a
used to monitor the output voltage. If V
is above its
OUT
programmedreferencethresholdnoswitchingoccursand
only quiescent current is drawn from the power source
final I
(I
OUT
is detected, and the part returns to sleep mode
(sleep mode). When V
drops below the reference
ZERO
OUT
is ignored and I
is used in all modes once
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.
VALLEY
ZERO
V
is above its programmed value).
4-Switch Mode
If (V – 400mV) < ~V < (V + 800mV), the LTC3531
OUT
OUT
IN
operates in 4-switch step-up/down mode. Returning to
Figure 1 (center) when V falls below its regulation volt-
In order to determine the best operating mode for the
converter, the LTC3531 contains a second comparator
OUT
age, switches A and C are turned on and current is ramped
until I is detected. As with Boost Mode operation, C
that monitors the relative voltage difference between V
PEAK
IN
is then turned off, D is turned on and current is delivered
to the output. When A and D are on, the inductor current
and V . Input and output voltages in the various modes
OUT
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.
slope is dependant on the relationship between V , V
,
IN OUT
and the R
of the switches. In 4-switch mode, a t
DSON
OFF
timer (approximately 3μs) is used to terminate the AD
pulse. Once the t timer expires, switch A is turned off,
OFF
B is turned on and inductor current is ramped down (V
Boost Mode
OUT
is applied across the inductor) until I
is detected.
VALLEY
If V is ~400mV below V , the LTC3531 operates in
IN
OUT
This sequence is repeated until the output is regulated,
BD switches are turned on, and a final I is detected.
boost or step-up mode. Referring to Figure 1 (left side)
when V falls below its regulation voltage, switches A
ZERO
OUT
Anticross conduction circuitry in all modes ensures the
P-channelMOSFETandN-channelMOSFETswitchpairs(A
and B or D and C) are never turned on simultaneously.
and C are turned on (V is applied across the inductor)
IN
and current is ramped until I
is detected. When this
PEAK
V
IN
V
OUT
V
IN
V
OUT
A
SW1
B
D
V
IN
L
SW2
C
I
MAX
I
PEAK
t
t
t
OFF
OFF
OFF
I
VALLEY
I
ZERO
AC AD AC AD
BOOST MODE
AC AD BD AC AD
4SW MODE
BD AC
AD
BD
AD BD AD BD
BUCK MODE
3531 F01
Figure 1. Voltage and Current Waveforms
3531fb
9