IR3476 datasheet(12/21 Pages) IRF

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August 8, 2012 | ADVANCED DATASHEET | V2.1 | PD97603

IR3476

12A Highly Integrated SupIRBuck

THEORY OF OPERATION

PWM COMPARATOR

The PWM comparator initiates a SET signal (PWM pulse)

when the FB pin falls below the reference (VREF) or the

soft start (SS) voltage.

ON-TIME GENERATOR

The PWM on-time duration is programmed with an

external resistor (RFF) from the input supply (VIN) to the FF

pin. The simplified equation for RFF is shown in equation 1.

The FF pin is held to an internal reference after EN goes

HIGH. A copy of the current in RFF charges a timing

capacitor, which sets the on-time duration, as shown in

equation 2.

CONTROL LOGIC

The control logic monitors input power sources, sequences

the converter through the soft-start and protective modes,

and initiates an internal RUN signal when all conditions are

met.

VCC and 3VCBP pins are continuously monitored, and the

IR3476 will be disabled if the voltage of either pin drops

below the falling thresholds. EN_DELAY will become HIGH

when VCC and 3VCBP are in the normal operating range

and the EN pin = HIGH.

SOFT START

With EN = HIGH, an internal 10µA current source charges

the external capacitor (CSS) on the SS pin to set the output

voltage slew rate during the soft start interval. The soft

start time (tSS) can be calculated from equation 3.

The feedback voltage tracks the SS pin until SS reaches the

0.5V reference voltage (Vref), then feedback is regulated

to Vref. CSS will continue to be charged, and when SS pin

reaches VSS (see Electrical Specification), SS_DELAY goes

HIGH. With EN_DELAY = LOW, the capacitor voltage and SS

pin is held to the FB pin voltage. A normal startup

sequence is shown in Figure 23.

PGOOD

The PGOOD pin is open drain and it needs to be externally

pulled high. High state indicates that output is in

regulation. The PGOOD logic monitors EN_DELAY,

SS_DELAY, and under/over voltage fault signals. PGOOD is

released only when EN_DELAY and SS_DELAY = HIGH and

output voltage is within the OV and UV thresholds.

PRE-BIAS STARTUP

IR3476 is able to start up into pre-charged output, which

prevents oscillation and disturbances of the output

voltage.

With constant on-time control, the output voltage is

compared with the soft start voltage (SS) or Vref,

depending on which one is lower, and will not start

switching unless the output voltage drops below the

reference. This scheme prevents discharge of a pre-biased

output voltage.

SHUTDOWN

The IR3476 will shutdown if VCC is below its UVLO limit.

The IR3476 can be shutdown by pulling the EN pin below

its lower threshold. Alternatively, the output can be

shutdown by pulling the soft start pin below 0.3V.

Figure 23: Normal Startup

(1)

OUT





(2)

V 





(3)







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