LTC3532 LINER [Linear Technology], LTC3532 Datasheet - Page 8

• Manufacturer Part Number: LTC3532
• Description: Micropower Synchronous Buck-Boost DC/DC Converter
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC3532
Buck Region (V
Switch D is always on and switch C is always off dur-
ing this mode. When the internal control voltage, V
above voltage V1, output A begins to switch. During the
off-time of switch A, synchronous switch B turns on for
the remainder of the time. Switches A and B will alternate
like 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 D
where D4
where f = operating frequency, Hz.
Beyond this point the “4-switch,” or buck/boost region
is reached.
Buck/Boost or 4-Switch (V
When the internal control voltage, V
V2, switch pair AD remain on for duty cycle D
and the switch pair AC begins to phase in. As switch pair
AC phases in, switch pair BD phases out accordingly.
When the V
range, at voltage V3, the AC switch pair completely phase
out the BD pair, and the boost phase begins at duty cycle
D4
begins is given by:
8
D
D4
SW
BOOST
BOOST
CYCLE
BUCK
D
D
DUTY
Figure 2. Switch Control vs Internal Control Voltage, V
D
MAX_BUCK
88%
MAX
MAX
MIN
0%
SW
. The input voltage, V
= (150ns • f) • 100 %
FOUR SWITCH PWM
PWM CD SWITCHES
PWM AB SWITCHES
SW
CI
A ON, B OFF
D ON, C OFF
= duty cycle % of the 4-switch range.
voltage reaches the edge of the buck/boost
= 100 – D4
IN
MAX_BUCK
U
> V
OUT
BOOST REGION
BUCK REGION
)
SW
, given by:
IN
IN
, where the 4-switch region
~ V
BUCK/BOOST REGION
%
OUT
)
CI
, is above voltage
3532 F02
V4 (≈2.05V)
V3 (≈1.65V)
V2 (≈1.55V)
V1 (≈0.9V)
INTERNAL
CONTROL
VOLTAGE, V
MAX_BUCK
CI
CI
CI
, is
,
The point at which the 4-switch region ends is given by:
Boost Region (V
Switch A is always on and switch B is always off dur-
ing this mode. When the internal control voltage, V
above voltage V3, switch pair CD will alternately switch
to provide a boosted output voltage. This operation is like
a synchronous boost regulator. The maximum duty cycle
of the converter is limited to 88% typical and is reached
when V
Burst Mode OPERATION
Burst Mode operation occurs when the IC delivers energy
to the output until it is regulated and then goes into a sleep
mode where the outputs are off and the IC is consuming
only 35μA of quiescent current from V
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 (47μF
or greater). Another method of reducing Burst Mode
operation ripple is to place a small feedforward capacitor
across the upper resistor in the V
network (as in Type III compensation). During the period
where the device is delivering energy to the output, the
peak switch current will be equal to 250mA typical and
the inductor current will terminate at zero current for each
cycle. In this mode the typical maximum average output
current is given by:
V
IN
V
I
OUT(MAX)BURST
IN
=
1– (150ns • f)
= V
CI
is above V4.
OUT
V
OUT
(1 – D) = V
IN
< V
≈
V
OUT
0.2 • V
OUT
OUT
)
+ V
(1 – 150ns • f) V
IN
IN
A
OUT
IN
. In this mode the
feedback divider
CI
3532fc
, is