IR3898MPBF datasheet(21/45 Pages) IRF

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AUGUST 08, 2012 | DATA SHEET | Rev 3.3

IR3898

6A Highly Integrated SupIRBuckTM

Single-Input Voltage, Synchronous Buck Regulator

PD-97662

FEEDFORWARD

Feed-Forward (F.F.) is an important feature, because it can

keep the converter stable and preserve its load transient

performance when Vin varies in a large range. In IR3898,

F.F. function is enabled when Vin pin is connected to PVin

pin. In this case, the internal low dropout (LDO) regulator is

used. The PWM ramp amplitude (Vramp) is proportionally

changed with Vin to maintain Vin/Vramp almost constant

throughout Vin variation range (as shown in Fig. 10). Thus,

the control loop bandwidth and phase margin can be

maintained constant. Feed-forward function can also

minimize impact on output voltage from fast Vin change.

The maximum Vin slew rate is within 1V/µs.

If an external bias voltage is used as Vcc, Vin pin should be

connected to Vcc/LDO_Out pin instead of PVin pin. Then

the F.F. function is disabled. A re-calculation of control

loop parameters is needed for re-compensation.

Vin

PWM Ramp

12V

Ramp Offset

21V

6.8V

12V

PWM Ramp

Amplitude = 1.8V

PWM Ramp

Amplitude = 3.15V

PWM Ramp

Amplitude = 1.02V

PWM Ramp

Amplitude = 0.15xVin

Figure 10: Timing Diagram for Feed-Forward (F.F.) Function

SMART LOW DROPOUT REGULATOR (LDO)

IR3898 has an integrated low dropout (LDO) regulator

which can provide gate drive voltage for both drivers.

In order to improve overall efficiency over the entire load

range, LDO voltage is set to 6.4V (typ.) at mid- or heavy

load condition to reduce Rds(on) and thus MOSFET

conduction loss; and it is reduced to 4.4 (typ.) at light load

condition to reduce gate drive loss.

The smart LDO can select its output voltage according to

the load condition by sensing switch node (SW) voltage. At

light load condition when part of the inductor current

flows in the reverse direction (DCM=1), VSW > 0 on LDrv

falling edge in a switching cycle. If this case happens for

consecutive 256 switching cycles, the smart LDO reduces

its output to 4.4. If in any one of the 256 cycles, Vsw < 0 on

LDrv falling edge, the counter is reset and LDO voltage

doesn’t change. On the other hand, if Vsw < 0 on LDrv

falling edge (DCM=0), LDO output is increased to 6.4V. A

hysteresis band is added to Vsw comparison to avoid

chattering. Figure 11a shows the timing diagram.

Whenever device turns on, LDO always starts with 6.4V,

then goes to 4.4V/6.4V depending upon the load

condition. For internally biased single rail operation, Vin

pin should be connected to PVin pin, as shown in Figure

11b. If external bias voltage is used, Vin pin should be

connected to Vcc/LDO_Out pin, as shown in Figure 11c.

Vcc/

LDO

256/Fs

...

6.4V

4.4V

...

Figure 11a: Time Diagram for Smart LDO

IR3898

VCC/

LDO_OUT

PGnd

Vin

PVin

Figure 11b: Internally Biased Single Rail Operation

Ext

VCC

Vin

IR3898

VCC/

LDO_OUT

PGnd

Vin

PVin

Figure 11c: Use External Bias Voltage

OUTPUT VOLTAGE TRACKING AND SEQUENCING

IR3898 can accommodate user programmable tracking

and/or sequencing options using Vp, Vref, Enable, and

Power Good pins. In the block diagram presented on page

3, the error-amplifier (E/A) has been depicted with three

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