LTC1435CS datasheet(7/20 Pages) LINER

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LTC1435

(Refer to Functional Diagram)

OPERATION

Main Control Loop

The LTC1435 uses a constant frequency, current mode

step-down architecture. During normal operation, the top

MOSFET is turned on each cycle when the oscillator sets

the RS latch, and turned off when the main current

comparator I1 resets the RS latch. The peak inductor

current at which I1 resets the RS latch is controlled by the

voltage on the ITHpin , which is the output of error amplifier

EA. The VOSENSE pin, described in the Pin Functions

section, allows EA to receive an output feedback voltage

VFB from an external resistive divider. When the load

current increases, it causes a slight decrease in VFB

relative to the 1.19V reference, which in turn causes the ITH

voltage to increase until the average inductor current

matches the new load current. While the top MOSFET is

off, the bottom MOSFET is turned on until either the

inductor current starts to reverse, as indicated by current

comparator I2, or the beginning of the next cycle.

The top MOSFET driver is biased from floating bootstrap

capacitor CB, which normally is recharged during each off

cycle. However, when VIN decreases to a voltage close to

VOUT, the loop may enter dropout and attempt to turn on

the top MOSFET continuously. The dropout detector counts

the number of oscillator cycles that the top MOSFET

remains on and periodically forces a brief off period to

allow CB to recharge.

The main control loop is shut down by pulling the RUN/SS

pin low. Releasing RUN/SS allows an internal 3

µA current

source to charge soft start capacitor CSS. When CSS

reaches 1.3V, the main control loop is enabled with the ITH

voltage clamped at approximately 30% of its maximum

value. As CSS continues to charge, ITH is gradually re-

leased allowing normal operation to resume.

Comparator OV guards against transient overshoots

> 7.5% by turning off the top MOSFET and keeping it off

until the fault is removed.

Low Current Operation

The LTC1435 is capable of Burst Mode operation in which

the external MOSFETs operate intermittently based on

load demand. The transition to low current operation

begins when comparator I2 detects current reversal and

turns off the bottom MOSFET. If the voltage across RSENSE

does not exceed the hysteresis of I2 (approximately 20mV)

for one full cycle, then on following cycles the top and

bottom drives are disabled. This continues until an induc-

tor current peak exceeds 20mV/RSENSE or the ITH voltage

exceeds 0.6V, either of which causes drive to be returned

to the TG pin on the next cycle.

Two conditions can force continuous synchronous opera-

tion, even when the load current would otherwise dictate

low current operation. One is when the common mode

voltage of the SENSE+ and SENSE – pins is below 1.4V and

the other is when the SFB pin is below 1.19V. The latter

condition is used to assist in secondary winding regulation

as described in the Applications Information section.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

of the other LTC1435 circuitry is derived from the INTVCC

pin. The bottom MOSFET driver supply pin is internally

connected to INTVCC in the LTC1435. When the EXTVCC

pin is left open, an internal 5V low dropout regulator

supplies INTVCC power. If EXTVCC is taken above 4.8V,

the 5V regulator is turned off and an internal switch is

turned on to connect EXTVCC to INTVCC. This allows the

INTVCC power to be derived from a high efficiency

external source such as the output of the regulator itself

or a secondary winding, as described in the Applications

Information section.

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