LTC1530CS8 datasheet(8/24 Pages) LINER

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LTC1530

G1 RISE/FALL

3300pF

0.1

µF

1530 F04a

PVCC

IFB

PVCC

12V

GND

G2 RISE/FALL

VOUT

COMP

LTC1530

3300pF

90%

tNOL

50%

COMP

1530 F04b

tSS

10%

Figure 4

OVERVIEW

The LTC1530 is a voltage feedback, synchronous switch-

ing regulator controller (see Block Diagram) designed for

use in high power, low voltage step-down (buck) convert-

ers. It includes an on-chip soft-start capacitor, a PWM

generator, a precision reference trimmed to

±1%,twohigh

power MOSFET gate drivers and all the necessary feed-

back and control circuitry to form a complete switching

regulator circuit running at 300kHz.

The LTC1530 includes a current limit sensing circuit that

uses the topside external N-channel power MOSFET as a

current sensing element, eliminating the need for an

external sense resistor. If the current comparator, CC,

detects an overcurrent condition, the duty cycle is reduced

by discharging the internal soft-start capacitor through a

voltage-controlled current source. Under severe over-

loads or output short-circuit conditions, the soft-start

capacitor is pulled to ground and a start-up cycle is

initiated. If the short circuit or overload persists, the chip

repeats soft-start cycles and prevents damage to external

components.

THEORY OF OPERATION

Primary Feedback Loop

The LTC1530 compares the output voltage with the inter-

nal reference at the error amplifier inputs. The error

amplifier outputs an error signal to the PWM comparator.

This signal is compared to the fixed frequency oscillator

sawtooth waveform to generate the PWM signal. The

PWM signal drives the external MOSFETs at the G1 and G2

pins. The resulting chopped waveform is filtered by LO and

COUT which closes the loop. Loop frequency compensa-

tion is typically accomplished with an external RC + C

network at the COMP pin, which is the output node of the

transconductance error amplifier.

MIN, MAX Feedback Loops

Two additional comparators in the feedback loop provide

high speed fault correction in situations where the error

amplifier cannot respond quickly enough. MIN compares

the feedback signal to a voltage 3% below the internal

reference. If the signal is below the comparator threshold,

the MIN comparator overrides the error amplifier and

forces the loop to maximum duty cycle, typically 86%.

Similarly, the MAX comparator forces the output to 0%

duty cycle if the feedback signal is greater than 3% above

the internal reference. To prevent these two comparators

from triggering due to noise, the MIN and MAX compara-

tors’ response times are deliberately delayed by two to

three microseconds. These comparators help prevent

extreme output perturbations with fast output load current

transients, while allowing the main feedback loop to be

optimally compensated for stability.

Thermal Shutdown

The LTC1530 has a thermal protection circuit that disables

both internal gate drivers if activated. G1 and G2 are held

low and the LTC1530 supply current drops to about 1mA.

TEST CIRCUITS

APPLICATIO S I FOR ATIO

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