XR76203 Datasheet, PDF (14/20 Pages) Exar Corporation – 3A/5A/8A Synchronous Step Down COT Regulator

XR76203/5/8
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-------fT}----^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=21.5mΩ, XR76205=59mΩ, XR76203=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 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 ini-
tiated 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_----_-----R_----D_-----S_----_----+----8_----m_-----V_--
ILIM
Where:
RLIM is resistor value for programming I_OCP
CSS
=
TSS



1_--0--0_-.--6-_--V-A_{---}
© 2015 Exar Corporation
14 / 20
exar.com/XR76203/5/8
Rev 1A