XRP7675 Datasheet, PDF (7/12 Pages) Exar Corporation – 3A 18V Synchronous PFM/PWM Step-Down Regulator

XRP7 6 7 5
3 A 1 8 V Sy n ch r on ou s PFM/ PW M St ep - Dow n Reg u lat or
THEORY OF OPERATION
FUNCTIONAL D ESCRIPTION
The XRP7675 is a synchronous, current - mode,
step - down regulator with light - load efficiency .
The light - load efficiency is achiev ed by
monitoring the current through M2 and turning
it off when current drops below 0A.
The
XRP7675 regulates input voltages from 4.5V to
18 V and supplies up to 3 A of load current . It
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 charge d from the internal 5V
supply when SW node is low. The XRP7675
has several powerful protection fe atures
including OCP, OVP, OTP, UVLO and output
short - circuit .
PROGRAMMABLE SOFT - START
The soft - start time is fully programmable via
CSS capacitor , placed between the SS and
GND pin . The CSS is charged by a 5µA
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
re alizing soft - start. Calculate the required CSS
from:
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Where:
tss is the required soft - start time
V_FB is the feedback voltage (0.925V nominal)
Please no te that the above is a simplified
equation and will provide an approximate CSS
value. For a required soft - start, a more
accurate CSS can be determined based on
empirical data.
OVERCURRENT PROTECTI ON AND HICCUP
MODE
The OCP protects against accidental increase
in load current or a short circuit. The current
of internal switch M1 is monitored. If this
current exceed s 5.6 A typical then a hiccup
mode is triggered . In hiccup mode, internal
power FETs are turned off and the SS pin is
discharged. When SS reaches 0.2V a softstar t
is initiated. The regul ator will stay in hiccup
mode until overcurrent is removed. Note that
wh en the soft start pin is below approximately
0.5V the regulator switching frequency is
90kHz.
OVERVOLTAGE PROTECTI ON OVP
The XRP7675 has internal OVP. When V _OUT
exceeds the OVP thre shold (when V _FB exceeds
1.1V) the power switching will be turned off .
The XRP7675 will restart when overvoltage
condition is removed .
OVER - TEMPERATURE PROTECTI ON OTP
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
XRP7675 will restart.
© 2013 Exar Corporation
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Rev. 1.0.0