XRP7665 Datasheet, PDF (7/12 Pages) Exar Corporation – 3A 18V Synchronous Step-Down Converter

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3 A 1 8 V Sy n ch r o n o u s St ep - Do w n Co n v er t er
THEORY OF OPERATION
FUNCTIONAL D ESCRIPTION
The XRP 7665 is a synchronous, current - mode,
step - down regulator. It regulates input
voltages from 4 .5V to 18V and supplies up to
3A of continuous load current . The XRP 7665
uses current - mode control to regulate the
output voltage. The output voltage is
measured at FB through a resistive voltage
divider and input to a transconductance error
amplifier. The high - side switch current is
compared to the output of the error amplifi
er
to control the output voltage.
The regulator
utilizes internal N - channel MOSFETs to step -
down the input voltage. A bootstrapping
capacitor connected between BS and SW acts
as a supply for high - side MOSFET. This
capacitor is charge d from the internal 5V
supply when SW node is low. The XRP 7665
has several powerful protection fe atures
including OCP, OVP, OTP, UVLO and output
short - circuit .
OVERCURRENT PROTECTI ON OCP
The OCP protects against accidental increase
in load current that can cause the regulator to
fail. The current of internal switch M1 is
monitored. If this current reaches
5.6 A then
M1 is turned off until next switch ing cycle.
SHORT - CIRCUIT PROTECTION
If th ere is short - circuit across the output , the
feedback voltage V _FB will droop. If V _FB drops
below 0.3V the XRP 7665 will detect a short -
circuit condition and reduce the switching
frequency to 90kHz for system protection.
The
regulator will restart once the short - circuit has
been removed.
OVERVOLTAGE PROTECTI ON OVP
The XRP 7665 has internal OVP. When V _OUT
exceeds the OVP threshold (when V _FB exceeds
1.1V) the power switching will be turned off .
The XRP 7665 will restart when overvoltage
condition is removed .
PROGRAMMABLE SOFT - START
OVE R- TEMPERATURE PROTECTI ON OTP
The soft - start time is fully programmable via
CSS capacitor , placed between the SS and
GND pin . The CSS is charged by a 6µA
constant - current source, generating a ramp
signal fed into non - inverting input of the error
amplifier. This ramp regulates the voltage on
comp pin during the regulator startup, thus
realizing soft - start. Calculate the required CSS
from:
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If the junction temperature exceeds 160
° C the
OTP circuit is triggered, turning off the internal
control circuit and switched M1 and M2.
When
junction temperature drops below 1 40° C the
XRP7665 will restart.
APPLICATION INFORM ATION
SETTING THE OUTPUT V OLTAGE
Where:
Use an external resistor divider to set the
tss is the required soft - start time
V_FB is the feedback voltage (0.925V nominal)
ENABLE FUNCTION
output voltage. Program the output voltage
from:
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The XRP7665 is enabled by raising the voltage
Where:
on the EN pin above 2.5V nominally. Connect
R1 is the resistor between V _OUT and FB
the EN pin to the
automatic start
pulling the EN
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and
GND
nominally.
0.925V is the nominal feedback voltage .
© 2013 Exar Corporation
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Rev. 2.0. 1