LTC3442 Datasheet, PDF (9/20 Pages) Linear Integrated Systems – Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation

U
OPERATIO
Reverse Current Limit
During fixed frequency operation, the LTC3442 operates
in forced continuous conduction mode. The reverse cur-
rent limit amplifier monitors the inductor current from
the output through switch D. Once the negative inductor
current exceeds 500mA typical, the IC will shut off switch D.
Four-Switch Control
Figure 1 shows a simplified diagram of how the four
internal switches are connected to the inductor, V_IN, V_OUT
and GND. Figure 2 shows the regions of operation for the
LTC3442 as a function of the internal control voltage, V_CI.
Depending on the control voltage, the IC will operate in
either buck, buck/boost or boost mode. The V_CI voltage is
a level shifted voltage from the output of the error amp (V_C)
(see Figure 5). The four power switches are properly
phased so the transfer between operating modes is con-
tinuous, smooth and transparent to the user. When V_IN
approaches V_OUT the buck/boost region is reached where
V_IN
V_OUT
9
8
PMOS A
SW1
4
PMOS D
SW2
6
NMOS B
NMOS C
3442 F01
Figure 1. Simplified Diagram of Output Switches
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
3442 F02 _CONTROL
VOLTAGE, V_CI
Figure 2. Switch Control vs Internal Control Voltage, V_CI
LTC3442
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.
Buck Region (V_IN > V_OUT)
Switch D is always on and switch C is always off during 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
similar to 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 DMAX_BUCK, given by:
DMAX_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 internal control voltage, V_CI, is above voltage V2,
switch pair AD remain on for duty cycle DMAX_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 four switch region begins
is given by:
V_IN
=
1–
V_OUT
(150ns
•
f)
The point at which the four switch region ends is given by:
V_IN = V_OUT(1 – D) = V_OUT(1 – 150ns • f) V
3442f
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