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LTC1435A

High Efficiency Low Noise Synchronous Step-Down Switching Regulator

Linear Technology 

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OPERATION (Refer to Functional Diagram)

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 3µA 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 released 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 LTC1435A 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 inductor current peak

exceeds 20mV/RSENSE or the ITH voltage exceeds 0.6V,

LTC1435A

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 LTC1435A circuitry is derived from the INTVCC

pin. The bottom MOSFET driver supply pin is internally

connected to INTVCC in the LTC1435A. 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.

APPLICATIONS INFORMATION

The basic LTC1435A application circuit is shown in Figure

1, High Efficiency Step-Down Converter. External compo-

nent selection is driven by the load requirement and begins

with the selection of RSENSE. Once RSENSE is known, COSC

and L can be chosen. Next, the power MOSFETs and D1 are

selected. Finally, CIN and COUT are selected. The circuit

shown in Figure 1 can be configured for operation up to an

input voltage of 28V (limited by the external MOSFETs).

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current. The

LTC1435A current comparator has a maximum threshold

of 150mV/RSENSE and an input common mode range of

SGND to INTVCC. The current comparator threshold sets

the peak of the inductor current, yielding a maximum av-

erage output current IMAX equal to the peak value less half

the peak-to-peak ripple current ∆IL.

Allowing a margin for variations in the LTC1435A and

external component values yields:

RSENSE

=

100mV

IMAX

The LTC1435A works well with RSENSE values ≥ 0.005Ω.

COSC Selection for Operating Frequency

The LTC1435A uses a constant frequency architecture with

the frequency determined by an external oscillator capaci-

tor COSC. Each time the topside MOSFET turns on, the

voltage COSC is reset to ground. During the on-time, COSC

is charged by a fixed current. When the voltage on the ca-

pacitor reaches 1.19V, COSC is reset to ground. The process

then repeats.

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