XR79115 Datasheet, PDF (13/19 Pages) Exar Corporation – 22V, 15A Synchronous Step Down COT Power Module

XR79115
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-----^-----^₂--^T--_.-₈-^O---₅-^N--_-–---1^---0²---_–⁵--₁--^-₀--1---0---^–---⁹---^--^
where T_ON is calculated from:
I_OCP is the over-current threshold to be programmed
RDS is the MOSFET rated On Resistance (5mΩ)
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 9). 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.
T_ON=
---------^V----^O----^U---^T----------
V_IN  f  Eff
where:
f is the desired switching frequency at nominal I_OUT
Eff is the Module efficiency corresponding to nominal I_OUT
shown in figures 19, 21, 23
Substituting for T_ON in the first equation we get:
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.
R_ON= ^---^{f--V----------O----E--U-------fT}----^f--^----₂^–---_.--₈^--^-₅--²--_-⁵--1--^-0--1-_–--0-₁--^–₀---⁹--^----^-----^V----^I-^N----^
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, then 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:
RLIM
=
_---I_--O_---C_----P_----_-----R_----D_-----S_----_----+----8_----m_-----V_--
ILIM
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:
CSS
=
TSS



1_--0--0_-.--6-_--V-A_{---}
Where:
RLIM is resistor value for programming I_OCP
Feed-Forward Capacitor (C_FF)
A feed-forward capacitor (C_FF) may be necessary depend-
ing on the Equivalent Series Resistance (ESR) of C_OUT. If
© 2014 Exar Corporation
13 / 19
exar.com/XR79115
Rev 1B