XR76208EL-Q Datasheet, PDF (14/21 Pages) Exar Corporation – AEC-Q100 Qualified 40V PowerBloxTM 3A/5A/8A Synchronous Step Down COT Regulator

XR76203-Q/XR76205-Q/XR76208-Q
Programming the On-Time
The On-Time T_ON is programmed via resistor R_ON accord-
ing to following equation:
R_ON = ^V-----^I-^N-----^-----^-3-^T--.-0-^O--5--^N--_-–--1-^--0-²--_–-⁵--₁--^₀---1---0---^–---⁹---^--^
where T_ON is calculated from:
T_ON=
---------^V-----^O---^U---^T----------
V_IN  f  Eff
where:
f is the desired switching frequency at nominal I_OUT
Eff is the Regulator efficiency corresponding to nominal
I_OUT shown in figures 19-24
Substituting for T_ON in the first equation we get:
R_ON= ^---^f---V-----^{----O----E--U--------Tf----f}-^-----3–---.-0^---^_5--²-_--⁵--1--^-0--1-_–--₁^0---₀^–---⁹---^---^-----^V----^I--^N----^
I_OCP is the over-current threshold to be programmed
RDS is the MOSFET rated On Resistance; XR76208-
Q=21.5mΩ, XR76205-Q=59mΩ, XR76203-Q=59mΩ
8mV is the OCP comparator maximum offset
ILIM is the internal current that generates the necessary
OCP comparator threshold (use 45μA).
Note that ILIM has a positive temperature coefficient of
0.4%/°C (figure 10). This is meant to roughly match and
compensate for positive temperature coefficient of the syn-
chronous FET. Graph of typical I_OCP versus RLIM is shown
in figure 7-9. Maximum allowable RLIM for XR76205-Q is
8.06kΩ.
Short-Circuit Protection (SCP)
If the output voltage drops below 60% of its programmed
value, the Module will enter hiccup mode. Hiccup will per-
sist until short-circuit is removed. SCP circuit becomes
active after PGOOD asserts high.
Over-Temperature (OTP)
OTP triggers at a nominal die temperature of 150°C. The
gate of switching FET and synchronous FET are turned off.
When die temperature cools down to 135°C, soft-start is
initiated and operation resumes.
Programming the Output Voltage
Use an external voltage divider as shown in the Application
Circuit to program the output voltage V_OUT.
Over-Current Protection (OCP)
If load current exceeds the programmed over-current, I_OCP,
for four consecutive switching cycles, the Module enters
hiccup mode of operation. In hiccup, the MOSFET gates
are turned off for 110ms (hiccup timeout). Following the hic-
cup timeout, a soft-start is attempted. If OCP persists, hic-
cup timeout will repeat. The Module will remain in hiccup
mode until load current is reduced below the programmed
I_OCP . In order to program the over-current protection, use
the following equation:
R1
=
R2



V----0-_{O--.-6U----T-}
–
1_
where R2 has a nominal value of 2kΩ.
Programming the Soft-start
Place a capacitor CSS between the SS and AGND pins to
program the soft-start. In order to program a soft-start time
of TSS, calculate the required capacitance CSS from the
following equation:
RLIM = ^---I--^O----^C---^P----^----IR--L--D--I--M-S---^----+-----8---m----V---
Where:
RLIM is resistor value for programming I_OCP
CSS
=
TSS



1--0-0--.-6-_{--V--A-- }
©2016-2017 Exar Corporation
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exar.com/XR76203-Q/XR705-Q/XR76208-Q
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