LTC1735 datasheet(10/32 Pages) LINER

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LTC1735

Foldback Current, Short-Circuit Detection and

Short-Circuit Latchoff

The RUN/SS capacitor, CSS, is used initially to limit the

inrush current of the switching regulator. After the con-

troller has been started and been given adequate time to

charge up the output capacitors and provide full load cur-

rent, CSS is used as a short-circuit time-out circuit. If the

output voltage falls to less than 70% of its nominal output

voltage, CSS begins discharging on the assumption that

the output is in an overcurrent and/or short-circuit condi-

tion. If the condition lasts for a long enough period as

determined by the size of CSS, the controller will be shut

down until the RUN/SS pin voltage is recycled. This built-

in latchoff can be overridden by providing a current >5

µA

at a compliance of 5V to the RUN/SS pin. This current

shortens the soft-start period but also prevents net dis-

charge of CSS during an overcurrent and/or short-circuit

condition. Foldback current limiting is activated when the

output voltage falls below 70% of its nominal level whether

or not the short-circuit latchoff circuit is enabled.

INTVCC/EXTVCC POWER

Power for the top and bottom MOSFET drivers and most

of the internal circuitry of the LTC1735 is derived from the

INTVCC pin. When the EXTVCC pin is left open, an internal

5.2V low dropout regulator supplies the INTVCC power

from VIN. If EXTVCC is raised above 4.7V, the internal

regulator is turned off and an internal switch connects

EXTVCC to INTVCC. This allows a high efficiency source,

such as the primary or a secondary output of the converter

itself, to provide the INTVCC power. Voltages up to 7V can

be applied to EXTVCC for additional gate drive capability.

To provide clean start-up and to protect the MOSFETs,

undervoltage lockout is used to keep both MOSFETs off

until the input voltage is above 3.5V.

OPERATIO

(Refer to Functional Diagram)

APPLICATIO S I FOR ATIO

The basic LTC1735 application circuit is shown in Figure 1

on the first page. External component 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. The

operating frequency and the inductor are chosen based

largely on the desired amount of ripple current. Finally, CIN

is selected for its ability to handle the large RMS current

into the converter and COUT is chosen with low enough

ESR to meet the output voltage ripple and transient speci-

fications. 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 LTC1735 current comparator has a maximum thresh-

old of 75mV/RSENSE and an input common mode range of

SGND to 1.1(INTVCC). The current comparator threshold

sets the peak of the inductor current, yielding a maximum

average output current IMAX equal to the peak value less

half the peak-to-peak ripple current,

∆IL.

Allowing a margin for variations in the LTC1735 and

external component values yields:

SENSE

MAX

COSC Selection for Operating Frequency and

Synchronization

The choice of operating frequency and inductor value is a

trade-off between efficiency and component size. Low

frequency operation improves efficiency by reducing

MOSFET switching losses, both gate charge loss and

transition loss. However, lower frequency operation re-

quires more inductance for a given amount of ripple

current.

The LTC1735 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 on COSC is reset to ground. During the on-time,

COSC is charged by a fixed current. When the voltage on the

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