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IR3548

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January 08, 2014

turned on, the SW voltage is driven to the VIN

supply voltage and the BOOT pin voltage is equal to

VIN plus the VDRV voltage without any bootstrap

diode voltage drop.

When the PWM signal goes low, the control

MOSFET is turned off by pulling the gate to the SW

voltage.

SYNCHRONOUS MOSFET DRIVER

The

IR3548

synchronous

MOSFET

driver

designed to drive the internal N-channel MOSFET to

frequencies of 1.5MHz. The driver is biased from the

VDRV supply voltage to turn the MOSFET on.

When the synchronous MOSFET is turned on the

SW node is pulled to ground. This allows recharging

of the bootstrap capacitor for the next synchronous

MOSFET drive event.

ADAPTIVE DEAD TIME ADJUSTMENT

In a synchronous buck converter care should be

taken to prevent both control and synchronous

MOSFETs from being on simultaneously. Such an

event could result in very large shoot-through

currents and lead to long term degradation of the

power stage. A fixed dead time does not provide

optimal performance due to variations in converter

duty cycles, bias voltages and temperature.

The

IR3548

provides

adaptive

dead

time

adjustment to minimize dead time to an optimum

duration which allows for maximum efficiency.

The ‘break before make’ adaptive design is achieved

by monitoring gate and SW voltages to determine

the ON or OFF status of a MOSFET. Adaptive

dead time also provides zero-voltage switching

(ZVS) of the synchronous MOSFET with minimum

current conduction through its body diode.

During

normal

operation

the

PWM

transitions

between low and high voltage levels to drive the

control and synchronous MOSFETs. The high

voltage level is 1.8V for IR ATL mode and 3.3V for

standard PWM mode.

The PWM signal falling edge transition to a low

voltage threshold initiates the control MOSFET

driver turn off after a short propagation delay. The

dead time control circuit monitors the internal high

gate signals and respective SW voltages to ensure

the control MOSFET is turned off before turning on

the synchronous MOSFET.

The PWM rising edge transition to a high voltage

threshold initiates the turn off of the synchronous

MOSFET after a small propagation delay. The

adaptive dead time circuit provides the appropriate

dead time by determining if the synchronous

MOSFET gate voltage has crossed the lower

threshold before allowing the turn on of the control

MOSFET.

INPUT VOLTAGE DIVIDER

The VINDIV can be used to provide input voltage

VIN1 information to the PWM controller with a ratio

of 1/14 through an internal resistor divider, which is

disconnected from VIN1 when EN1 is low to reduce

the bias current of the IR3548.

FREQUENCY RANGE

The IR3548 is designed to operate over a wide

frequency range. When operating in Dual Mode, the

PWM input and respective SW output frequencies

are identical. When operating in Doubler mode, the

PWM1 input frequency is twice the SW1 or SW2

output frequencies. When operating in Quad mode,

the Master PWM1 input frequency is four times the

four SW output frequencies.

The lower limit of the output frequency range is

dictated by the size of the BOOT capacitor which

provides bias to the control MOSFET driver during

the entire on-time. The upper limit of frequency is

determined by thermal limitations as well as pulse

width limitations. The IR3548 is designed to operate

with switching frequencies of each phase between

200kHz and 1.5MHz.

USING A DUAL MODE DRIVER AS A SINGLE

DRIVER

To use the IR3548 in a single phase converter,

simply set Dual mode, ground EN2 pin and leave the

unused PWM2 input floating, as shown in Figure 5.

DUAL MODE REACTION TO TRI-STATE

PWM INPUT

Anytime there is a tri-state on the PWM1 or PWM2,

the respective SW output is tri-stated.

IR3548 Datasheet(PDF) 21 Page - International Rectifier

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