PCMB065T-2R2MS datasheet(19/47 Pages) IRF

19 / 47 page

Dual output, 4A/Phase, Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

IR3891

www.irf.com

May 08, 2013

Each channel of the IR3891 can be shutdown

separately by pulling the channel EN pin below its low

threshold. Each EN pin controls only one channel to

allow the user to operate each independently.

OVER CURRENT PROTECTION (CURRENT LIMIT

AND HICCUP MODE)

The over-current protection is performed by sensing

current through the RDS(on) of the Sync FET. This

method enhances the converter’s efficiency and

reduces cost by eliminating a current sense resistor.

The current limit is pre-set internally and compensated

to maintain an almost constant limit over temperature.

IR3891 determines over-current events when the

Synchronous FET is on. OCP circuit samples this

current for 40 nsec typically after the rising edge of the

PWM set pulse which has a width of 12.5% of the

switching period. The PWM pulse starts at the falling

edge of the PWM set pulse.

This makes valley

current sense more robust as current is sensed close

to the bottom of the inductor downward slope where

transient and switching noise are lower and helps to

prevent false tripping due to noise and transient. An

OC condition is detected if the load current exceeds

the threshold, the converter enters into hiccup mode.

PGood will go low and the internal soft start signal will

be pulled low.

LIMIT

OCP







(2)

IOCP

= DC current limit hiccup point

ILIMIT

= Current Limit Valley Point

∆i

= Inductor ripple current

Hiccup mode is when the converter stops and waits

before restarting. The channel waits for Tblk_Hiccup,

2.48 ms typical, before the OC signal resets and

restarts. In normal application, the converter restarts

with a pre-bias sequence and soft-start. Figure 11

shows the timing diagram of the above OC protection.

If another OC event is detected, the part repeats

hiccup mode.

Figure 11: Timing diagram for pulse-by-pulse current

limit and Hiccup mode

THERMAL SHUTDOWN

IR3891 provides thermal protection. A thermal fault is

detected, when the temperature of the part reaches

the Thermal Shutdown Threshold, 145°C typical. A

thermal fault results in both channels turning off. The

power MOSFETs are disabled during thermal

shutdown.

IR3891 automatically restarts when the

temperature of the part drops back below the lower

thermal limit, typically 20°C below the Thermal

Shutdown Threshold.

FEED-FORWARD

Feed-Forward is an important feature which helps with

stability and preserves load transient performance

during PVin changes. In IR3891, Feed-Forward (F.F.)

function is enabled when Vin pin is connected to PVin

pin and Vin>5.0V. The PWM ramp amplitude (Vramp)

is proportionally changed with respect to Vin to

maintain PVin/Vramp ratio.

The ratio is almost

constant throughout the Vin range (as shown in Figure

12).

By maintaining a constant PVin/Vramp, the

control loop bandwidth and phase margin are more

constant. F.F. function also helps minimize the effect

of PVin changes on the output voltage.

Feed-Forward is based on the Vin voltage and needs

to be accounted for when calculating IR3891

compensation.

The PVin/Vramp ratio is not

maintained when Vin and PVin are not equal. This is

the case when an external bias voltage for VCC.

When using an external VCC voltage, Vin pin should

be connected to the VCC pin instead of the PVin pin.

Compensation for the configuration should reflect the

separation.

PCMB065T-2R2MS Datasheet(PDF) 19 Page - International Rectifier

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