LTC3444_15 Datasheet, PDF (10/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

LTC3444
U
OPERATIO
Buck-Boost Four-Switch Control
Figure 3 shows a simplified diagram of how the four
internal switches are connected to the inductor, V_IN, V_OUT
and GND. Figure 4 shows the regions of operation for the
LTC3444 as a function of the internal control voltage, V_CI.
Depending on the control voltage, the LTC3444 will oper-
ate 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 pin) (see Figure 2). The four power switches
are properly phased so the transfer between operating
modes is continuous, smooth and transparent to the user.
The buck-boost region is reached when V_IN approaches
V_OUT. The conduction time of the four switch region is
typically 125ns. The three operating modes of the four
switch buck-boost converter are described below. Please
refer to Figures 3 and 4.
V_IN
6
PMOS A
SW1
2
NMOS B
V_OUT
5
PMOS D
SW2
4
NMOS C
3444 F03
Figure 3. 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
0%
DUTY
CYCLE
V4 (~1.16V)
V3 (~0.73V)
V2 (~0.49V)
V1 (OV)
INTERNAL
CONTROL
VOLTAGE, V_CI
3444 F04
Figure 4. Switch Control vs Internal Control Voltage, V_CI
3444fb
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