SL40307A-R19KHF datasheet(24/53 Pages) IRF

24 / 53 page

IR3846

24 www.irf.com

August 01, 2013

Figure 19: Externally Biased Configuration

When the Vin voltage is below 6.8V, the internal LDO

enters the dropout mode at medium and heavy load.

The dropout voltage increases with the switching

frequency. Figure 20 shows the LDO voltage for

600kHz

and

1000kHz

switching

frequency

respectively.

Figure 20: LDO_Out Voltage in dropout mode

OUTPUT VOLTAGE TRACKING AND

SEQUENCING

IR3846

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 positive inputs. Ideally, the

input with the lowest voltage is used for regulating the

output voltage and the other two inputs are ignored. In

practice the voltage of the other two inputs should be

at least 200mV greater than the low-voltage input so

that their effects can completely be ignored. Vp is

pulled up to an internal rail via a high impedance path.

For normal operation, Vp and Vref is left floating (Vref

should have a bypass capacitor).

Therefore, in normal operating condition, after Enable

goes high, the internal soft-start (Intl_SS) ramps up

the output voltage until Vfb (voltage of feedback/Fb

pin) reaches about 0.6V. Then Vref takes over and the

output voltage is regulated.

Tracking-mode operation is achieved by connecting

Vref to LGND. Then, while Vp = 0V, Enable is taken

above its threshold so that the soft-start circuit

generates Intl_SS signal. After the Intl_SS signal

reaches the final value (refer to Figure 7), ramping up

the Vp input will ramp up the output voltage. In

tracking mode, Vfb always follows Vp which means

Vout is always proportional to Vp voltage (typical for

DDR/Vtt rail applications). The effective Vp range is

0V~1.2V.

In sequencing mode of operation (simultaneous or

ratiometric), Vref is left floating and Vp is kept to

ground level until Intl_SS signal reaches the final

value. Then Vp is ramped up and Vfb follows Vp.

When Vp>0.6V the error-amplifier switches to Vref

and the output voltage is regulated with Vref. The

final Vp voltage after sequencing startup should

between 0.8V ~ 3.0V.

Figure 21: Typical waveforms for sequencing mode of

operation: (a) simultaneous, (b) ratiometric

SL40307A-R19KHF 数据表(PDF) 24 Page - International Rectifier

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