LTC3534 datasheet(13/34 Pages) AD

13 / 34 page

LTC3114-1

Rev. D

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OPERATION

Oscillator

The LTC3114-1 operates from an internal oscillator with

a nominal fixed frequency of 1.2MHz. This allows the

DC/DC converter efficiency to be maximized while still

using small external components.

Current Mode Control

The LTC3114-1 utilizes average current mode control for

the pulse width modulator as shown in Figure 2. Current

mode control, both average and the better known peak

method, enjoy some benefits compared to other control

methods including: simplified loop compensation, rapid

response to load transients and improved rejection of line

voltage transients.

Referring to Figure 2, an internal high gain transconduc-

tance error amplifier monitors VOUT through a voltage

divider connected to the FB pin and provides an output,

VC, that is used by the current mode control loop to com-

mand the appropriate inductor current level. To ensure

stability, external frequency compensation components

(CP1, CP2 and RZ) must be installed between VC and

ground. The procedure for determining these components

is provided in the Applications Information section of this

data sheet. VC is internally connected to the noninvert-

ing input of a high gain, integrating, operational amplifier,

referred to in Figure 2, as the average current amp. The

inverting input of the average current amplifier is con-

nected to the inductor current sense circuit through a gain

setting resistor (RCS1) and to its output (VIA) through

an internal frequency compensation network comprised

of RCS2, CCS1 and CCS2. The average current amplifier’s

output provides the cycle-by-cycle duty cycle command

into the buck-boost PWM circuitry.

PWM

A B C D

CONTROL

LOGIC

VIA

AVERAGE

CURRENT

AMP

VOLTAGE

ERROR

AMP

ACTIVE RANGE OF

VC = 135mV TO 1V

SOFT-START

RCS1

CCS2

RCS2

CCS1

RBOT

RTOP

CP1

CP2

RCOESR

RLOAD

VOUT

COUT

gm = 111µA/V

VIN

SWB

SWC

SWD PVOUT

SWA

250m

VDS

SENSE

VDS

SENSE

gm = 120µS

RO = 3.6M

GND

31141 F02

INDUCTOR

ISENSE

Figure 2. Average Current Mode Control Loop

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