LTC3532_15 Datasheet, PDF (8/16 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3532
U
OPERATIO
88%
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 (≈2.05V)
V3 (≈1.65V)
V2 (≈1.55V)
0%
V1 (≈0.9V)
DUTY
CYCLE
INTERNAL
3532 F02 _CONTROL
VOLTAGE, V_CI
Figure 2. Switch Control vs Internal Control Voltage, V_CI
Buck Region (V_IN > V_OUT)
Switch D is always on and switch C is always off dur-
ing this mode. When the internal control voltage, V_CI, is
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_{MAX_BUCK}, given by:
D_{MAX_BUCK} = 100 – D4_SW %
where D4_SW = duty cycle % of the 4-switch range.
D4_SW = (150ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the “4-switch,” or buck/boost region
is reached.
Buck/Boost or 4-Switch (V_IN ~ V_OUT)
When the internal control voltage, V_CI, is above voltage
V2, switch pair AD remain on for duty cycle D_{MAX_BUCK},
and the switch pair AC begins to phase in. As switch pair
AC phases in, switch pair BD phases out accordingly.
When the V_CI voltage reaches the edge of the buck/boost
range, at voltage V3, the AC switch pair completely phase
out the BD pair, and the boost phase begins at duty cycle
D4_SW. The input voltage, V_IN, where the 4-switch region
begins is given by:
V_IN
=
1–
V_OUT
(150ns
•
f)
The point at which the 4-switch region ends is given by:
V_IN = V_OUT(1 – D) = V_OUT(1 – 150ns • f) V
Boost Region (V_IN < V_OUT)
Switch A is always on and switch B is always off dur-
ing this mode. When the internal control voltage, V_CI, is
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_CI is above V4.
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_IN. In this mode the
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_OUT feedback divider
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:
I_{O U T(M A X )B U R S T}
≈
0.2 • V_IN
V_OUT + V_IN
A
3532fc
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