XRP7665
3A 18V Synchronous Step-Down Converter
THEORY OF OPERATION
OVERCURRENT PROTECTION OCP
The OCP protects against accidental increase
in load current that can cause the regulator to
FUNCTIONAL DESCRIPTION
fail. The current of internal switch M1 is
monitored. If this current reaches 5.6A then
M1 is turned off until next switching cycle.
The XRP7665 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 XRP7665
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 amplifier
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 charged from the internal 5V
supply when SW node is low. The XRP7665
has several powerful protection features
including OCP, OVP, OTP, UVLO and output
short-circuit.
SHORT-CIRCUIT PROTECTION
If there is short-circuit across the output, the
feedback voltage VFB will droop. If VFB drops
below 0.3V the XRP7665 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 PROTECTION OVP
The XRP7665 has internal OVP. When VOUT
exceeds the OVP threshold (when VFB exceeds
1.1V) the power switching will be turned off.
The XRP7665 will restart when overvoltage
condition is removed.
PROGRAMMABLE SOFT-START
OVER-TEMPERATURE PROTECTION 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:
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 140°C the
XRP7665 will restart.
APPLICATION INFORMATION
6μ
=
×
SETTING THE OUTPUT VOLTAGE
Where:
Use an external resistor divider to set the
output voltage. Program the output voltage
from:
tss is the required soft-start time
VFB is the feedback voltage (0.925V nominal)
1 = 2 ×
− 1
0.9 2 5
ENABLE FUNCTION
Where:
R1 is the resistor between VOUT and FB
The XRP7665 is enabled by raising the voltage
on the EN pin above 2.5V nominally. Connect
the EN pin to the VIN via a 1 0 0 k Ω r e s i s t o r
f o r
R2 is the resistor between FB and GND
automatic start-up. Shutdown is achieved by
pulling the EN pin voltage below 1.1V
nominally.
( n o mi n a l l y 1 0 k Ω)
0.925V is the nominal feedback voltage.
7/12 Rev. 2.0.1
© 2013 Exar Corporation
EXAR [ EXAR CORPORATION ]
