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LTC3442EDE-TRPBF

Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation

Linear Technology 

LTC3442

OPERATION

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 in-

ternal switches are connected to the inductor, VIN, VOUT

and GND. Figure 2 shows the regions of operation for the

LTC3442 as a function of the internal control voltage, VCI.

Depending on the control voltage, the IC will operate in

either buck, buck/boost or boost mode. The VCI voltage

is a level shifted voltage from the output of the error amp

(VC) (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 VIN

approaches VOUT the buck/boost region is reached where

VIN

VOUT

9

8

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 (VIN > VOUT)

Switch D is always on and switch C is always off during

this mode. When the internal control voltage, VCI, 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 – D4SW %

where D4SW = duty cycle % of the four switch range.

D4SW = (150ns • f) • 100 %

where f = operating frequency, Hz.

Beyond this point the “four switch,” or buck/boost region

is reached.

PMOS A

SW1

4

NMOS B

PMOS D

SW2

6

NMOS C

3442 F01

Figure 1. Simplified Diagram of Output Switches

88%

DMAX

BOOST

DMIN

BOOST

DMAX

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%

V4 (≈2.05V)

V3 (≈1.65V)

V2 (≈1.55V)

V1 (≈0.9V)

Buck/Boost or Four Switch (VIN ~ VOUT)

When the internal control voltage, VCI, 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 VCI 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

D4SW. The input voltage, VIN, where the four switch region

begins is given by:

VIN

=

1–

VOUT

(150ns

•

f)

The point at which the four switch region ends is given by:

VIN = VOUT(1 – D) = VOUT(1 – 150ns • f) V

DUTY

CYCLE

INTERNAL

3442 F02 CONTROL

VOLTAGE, VCI

Figure 2. Switch Control vs Internal Control Voltage, VCI

For more information www.linear.com/LTC3442

3442fb

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