LTC3727A-1 Datasheet, PDF(19/32 Page) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

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LTC3727A-1 Datasheet, PDF (19/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

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LTC3727A-1

APPLICATIO S I FOR ATIO

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 LTC3727A-1. Soft-start reduces the input power

sourceâs surge currents by gradually increasing the

controllerâs current limit (proportional to VITH). This pin

can also be used for power supply sequencing.

An internal 1.2ÂµA 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 45mV/

RSENSE to 135mV/RSENSE. The output current limit ramps

up slowly, taking an additional 1.25s/ÂµF 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

1.2ÂµA

CSS

(1.25s

ÂµF)CSS

tIRAMP

3V â 1.5V

1.2ÂµA

CSS

(1.25s

ÂµF)CSS

By pulling both RUN/SS pins below 1V, the LTC3727A-1

is put into low current shutdown (IQ = 20ÂµA). The RUN/SS

pins can be driven directly from logic as shown in Fig-

ure 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 Functional Diagram).

Fault Conditions: Current Limit and Current Foldback

The LTC3727A-1 current comparator has a maximum

sense voltage of 135mV resulting in a maximum MOSFET

current of 135mV/RSENSE. The maximum value of current

limit generally occurs with the largest VIN at the highest

ambient temperature, conditions that cause the highest

power dissipation in the top MOSFET.

The LTC3727A-1 includes current foldback to help further

limit load current when the output is shorted to ground.

The foldback circuit is active even when the overload

shutdown latch described above is overridden. If the

output falls below 70% of its nominal output level, then the

maximum sense voltage is progressively lowered from

135mV to 45mV. Under short-circuit conditions with very

low duty cycles, the LTC3727A-1 will begin cycle skipping

in order to limit the short-circuit current. In this situation

the bottom MOSFET will be dissipating most of the power

but less than in normal operation. The short-circuit ripple

current is determined by the minimum on-time tON(MIN) of

the LTC3727A-1 (less than 200ns), the input voltage and

inductor value:

âIL(SC) = tON(MIN) (VIN/L)

The resulting short-circuit current is:

ISC

45mV

RSENSE

âIL(SC)

Fault Conditions: Overvoltage Protection (Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the regulator rises

much higher than nominal levels. The crowbar causes

huge currents to flow, that blow the fuse to protect against

a shorted top MOSFET if the short occurs while the

controller is operating.

3.3V OR 5V

RUN/SS

CSS

(7a)

Figure 7.

RUN/SS

(7b)

CSS

3727 F07

3727a1f

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