XRP7662 Datasheet, PDF (10/19 Pages) Exar Corporation – 12A 300KHz Synchronous Step Down Regulator

XRP7 66 2
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1 2 A 3 0 0 KHz Sy n ch r on ou s St ep Dow n Reg u lat or
the V _IN start threshold, use a pair of external
resistors as shown. I t is recommended to use
external resistors an order of magnitude
smaller than internal resistors, then the V
IN
start threshold is given by:
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For example, if it is required to have a V
IN
start threshold of 7V, then let R ₇ = 5k Æ» DQG
using the V _IN start threshold equation we get
R₆ = 9.09 kÆ»
THERMAL AND SHORT - CIRCUIT PROTECTION
Fig. 19 : Over Current Detection Circuit
Inputs to an over -current detection
Because the XRP7662 is designed to drive
comparator, set to trigger at 60mV nominal,
large output current, there is a chance that
are connected to the inductor as shown. Since
the power converter will become too hot.
the average voltage sensed by the comparator
Therefore, an internal thermal shutdown
is equal to the product of inductor current and
(145°C) has been included to prevent the IC
inductor DC resistance (DCR), then:
from malfunctioning at extreme tem
peratures.
A short - circuit de tection comparator has also
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been included in the XRP7662 to protect
against an accidental short at the output of
the power converter. This comparator con
-
stantly monitors the positive and negative
terminals of the error amplifier, and if the VFB
pin falls more than 250mV (typical) below the
positive reference, a short - circuit fault is set.
Because the SS pin overrides the internal 0.8V
Solving this equation for the specific inductor
in circuit 1, I _MAX= 14.6 A. When I _MAX is reached,
a 220ms time - out is initiated, during which top
and bottom drivers are turned off. Following
the time - out, a restart is attempted. I f the
fault condition persists, then the time
- out is
repeated (referred to as hiccup).
reference during soft start, the
XRP7662 is
capable of detecting short -circuit faults
Increasing the Current Limit
throughout the duration of soft start
as well as
If it is desired to set I _MAX > (60mV/DCR) (in
in regular operation.
this case larger than 14.6A), then a resistor R
9
should be added as shown in Figure
20 . R ₉
OVER - CURRENT PROTECTION
for ms a resistor divider and reduces the
voltage seen by the comparator.
The Over - current protection feature can only
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limited by the common mode rating of the op
-
amp used to sense the voltage
across the
inductor. Over -current is detected by
monitoring a differential voltage across t
he
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Solving for R ₉ we get:
output i nductor as shown in the Figure 1 9.
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As an example: if desired
R₉ = 63 .4k Æ»
I _MAX is 17 A, then
© 2012 Exar Corporation
10 / 19
Rev. 2. 2.0