NB638 Datasheet, PDF (4/19 Pages) Monolithic Power Systems – High Efficiency, Fast Transient, 7A, 28V Synchronous Step-down Converter in a Tiny 3x4mm QFN Package

NB638–HIGH EFFICIENCY, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER
PIN FUNCTIONS
Pin #
1
Name
AGND
2
FREQ
3
FB
4
SS
5
EN
6
PGOOD
7
BST
8, 19
9, 10, 17, 18
11-16
20
IN
SW
PGND
VCC
Description
Analog Ground.
Frequency Set during CCM operation. The ON period is determined by the input
voltage and the frequency-set resistor connected to FREQ pin. Connect a resistor
to IN for line feed-forward. Decouple with a 1nF capacitor.
Feedback. An external resistor divider from the output to GND, tapped to the FB
pin, sets the output voltage.
Soft Start. Connect an external SS capacitor to program the soft start time for the
switch mode regulator. When the EN pin becomes high, an internal current source
(8.5µA) charges up the SS capacitor and the SS voltage slowly ramps up from 0 to
V_FB smoothly. When the EN pin becomes low, an internal current source (8.5μA)
discharges the SS capacitor and the SS voltage slowly ramps down.
EN=1 to enable the NB638. For automatic start-up, connect EN pin to IN with a
100kΩ resistor. It includes an internal 1MΩ pull-down resistor.
Power Good Output. The output of this pin is an open drain and is high if the
output voltage is higher than 90% of the nominal voltage. There is delay from FB ≥
90% to PGOOD high, which is 50% of SS time plus 0.5ms.
Bootstrap. A 0.1µF-1µF capacitor connected between SW and BS pins is required
to form a floating supply across the high-side switch driver.
Supply Voltage. The NB638 operates from a +4.5V to +28V input rail. C_IN is
needed to decouple the input rail. Use wide PCB traces and multiple vias to make
the connection.
Switch Output. Use wide PCB traces and multiple vias to make the connection.
System Ground. This pin is the reference ground of the regulated output voltage.
For this reason care must be taken in PCB layout.
Internal Bias Supply. Decouple with a 1µF capacitor as close to the pin as
possible.
NB638 Rev.1.16
www.MonolithicPower.com
4
4/18/2012
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