LTC3538 Datasheet, PDF (9/16 Pages) Linear Technology – 800mA Synchronous Buck-Boost DC/DC Converter

LTC3538
OPERATION
Depending on the V_C voltage, the LTC3538 will operate in
either buck, buck-boost or boost mode. The four power
switches are properly phased so the transfer between
operating modes is continuous, smooth and transparent to
the user. When V_IN approaches V_OUT the buck-boost region
is entered, where the conduction time of the four-switch
region is typically 150ns. Referring to Figures 1 and 2, the
various regions of operation will now be described.
88%
D_MAX
BOOST
A ON, B OFF
PWM C, D SWITCHES
D_MIN
BOOST
FOUR-SWITCH PWM
D_MAX
BUCK
D ON, C OFF
PWM A, B SWITCHES
BOOST REGION
BUCK-BOOST
REGION
BUCK REGION
V4 (~2.2V)
V3 (~1.8V)
V2 (~1.7V)
0%
V1 (~1.2V)
DUTY
CYCLE
CONTROL
3538 F02 _{VOLTAGE, VC}
Figure 2. Switch Control vs Control Voltage, V_C
Buck Region (V_IN > V_OUT)
Switch D is always on and switch C is always off during
this mode. When the control voltage, V_C, is above volt-
age V1, output A begins to switch. During the off time of
switch A, synchronous switch B turns on for the remainder
of the period. Switches A and B will alternate similar to a
typical synchronous buck regulator. As the control volt-
age 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 four switch range.
D4_SW = (150ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the four switch, or buck-boost region
is reached.
Buck-Boost or Four Switch (V_IN ~ V_OUT)
When the control voltage, V_C, is above voltage V2, switch
pair AD remains 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 V_C 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 four switch region begins is given by:
V_IN = V_OUT(1 – D4_SW) ≈ 0.85 • V_OUT
The point at which the four-switch region ends is given
by:
V_IN
=
V_OUT
1− D4_SW
V
≈ 1.18 • V_OUT
Boost Region (V_IN < V_OUT)
Switch A is always on and switch B is always off during
this mode. When the control voltage, V_C, is above volt-
age V3, switch pair CD will alternately switch to provide
a boosted output voltage. This operation is typical to a
synchronous boost regulator. The maximum duty cycle
of the converter is limited to 88% typical and is reached
when V_C is above V4.
Burst Mode OPERATION
Burst Mode operation reduces quiescent current consump-
tion of the LTC3538 at light loads and improves overall
conversion efficiency, increasing battery life. During Burst
Mode operation the LTC3538 delivers energy to the out-
put until it is regulated and then goes into a sleep mode
where the outputs are off and the quiescent current drops
to 35μA. 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. Another method of reducing Burst
Mode operation ripple is to place a small feed-forward
capacitor across the upper resistor in the V_OUT feedback
divider network (as in Type III compensation).
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