IR3847 Datasheet, PDF (26/53 Pages) International Rectifier – Single-Input Voltage, Synchronous Buck Regulator 25A Highly Integrated SupIRBuck

IR3847
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.
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 20: LDO_Out Voltage in dropout mode
OUTPUT VOLTAGE TRACKING AND
SEQUENCING
IR3847 can accommodate user programmable
tracking and/or sequencing options using Vp, Vref,
Enable, and Power Good pins. In the block diagram
presented on page 4, 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.
26 www.irf.com © 2013 International Rectifier
Figure 21: Typical waveforms for sequencing mode of
operation: (a) simultaneous, (b) ratiometric
May 20, 2013