LTC3707-SYNC 查看數據表（PDF） - Linear Technology

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LTC3707-SYNC

High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

Linear Technology 

LTC3707-SYNC

APPLICATIO S I FOR ATIO

R1(MAX)

=



24k

0.8V

2.4V – VOUT





for VOUT < 2.4V

Regulating an output voltage of 1.8V, the maximum value

of R1 should be 32K. Note that for an output voltage above

2.4V, R1 has no maximum value necessary to absorb the

sense currents; however, R1 is still bounded by the

VOSENSE feedback current.

Soft-Start/Run Function

The RUN/SS1 and RUN/SS2 pins are multipurpose pins

that provide a soft-start function and a means to shut

down the IC. Soft-start reduces the input power source’s

surge currents by gradually increasing the controller’s

current limit (proportional to VITH). This pin can also be

used for power supply sequencing.

An internal 1.2µA current source charges up the CSS

capacitor. When the voltage on RUN/SS1 (RUN/SS2)

reaches 1.5V, the particular controller is permitted to start

operating. As the voltage on RUN/SS increases from 1.5V

to 3.0V, the internal current limit is increased from 25mV/

RSENSE to 75mV/RSENSE. The output current limit ramps

up slowly, taking an additional 1.25s/µF to reach full

current. The output current thus ramps up slowly, reduc-

ing the starting surge current required from the input

power supply. If RUN/SS has been pulled all the way to

ground there is a delay before starting of approximately:

( ) tDELAY

=

1.5V

1.2µA

C SS

=

1.25s / µF

C SS

( ) tIRAMP

=

3V − 1.5V

1.2µA

C SS

=

1.25s / µF

C SS

By pulling both RUN/SS pins below 1V, the IC is put into

low current shutdown (IQ = 20µA). The RUN/SS pins can

be driven directly from logic as shown in Figure 7. Diode

D1 in Figure 7 reduces the start delay but allows CSS to

ramp up slowly providing the soft-start function. Each

RUN/SS pin has an internal 6V zener clamp (See Func-

tional Diagram).

3.3V OR 5V

D1

VIN

RUN/SS

RSS*

CSS

INTVCC

RSS*

RUN/SS

CSS

*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF

(a)

(b)

3707 F07

Figure 7. RUN/SS Pin Interfacing

Fault Conditions: Overcurrent Latchoff

The RUN/SS pins also provide the ability to latch off the

controller(s) when an overcurrent condition is detected.

The RUN/SS capacitor, CSS, is used initially to turn on and

limit the inrush current. After the controller has been

started and been given adequate time to charge up the

output capacitor and provide full load current, the RUN/SS

capacitor is used for a short-circuit timer. If the regulator’s

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

after CSS reaches 4.1V, CSS begins discharging on the

assumption that the output is in an overcurrent condition.

If the condition lasts for a long enough period as deter-

mined by the size of the CSS and the specified discharge

current, the controller will be shut down until the RUN/SS

pin voltage is recycled. If the overload occurs during start-

up, the time can be approximated by:

tLO1 ≈ [CSS (4.1 – 1.5 + 4.1 – 3.5)]/(1.2µA)

= 2.7 • 106 (CSS)

If the overload occurs after start-up the voltage on CSS will

begin discharging from the zener clamp voltage:

tLO2 ≈ [CSS (6 – 3.5)]/(1.2µA) = 2.1 • 106 (CSS)

This built-in overcurrent latchoff can be overridden by

providing a pull-up resistor to the RUN/SS pin as shown

in Figure 7. This resistance shortens the soft-start period

and prevents the discharge of the RUN/SS capacitor

during an over current condition. Tying this pull-up resis-

tor to VIN as in Figure 7a, defeats overcurrent latchoff.

Diode-connecting this pull-up resistor to INTVCC, as in

Figure 7b, eliminates any extra supply current during

controller shutdown while eliminating the INTVCC loading

from preventing controller start-up.

3707sf

19

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