XRP6141 Datasheet, PDF (11/14 Pages) Exar Corporation – 35A Synchronous Step Down COT Controller

XRP6141
35A Synchronous Step Down COT Controller
Note that switching frequency f will increase somewhat, as
a function of increasing load current and increasing losses
(see figure 22).
OVER-CURRENT PROTECTION (OCP)
If load current exceeds the programmed over-current IOCP
for four consecutive switching cycles, then IC enters hiccup
mode of operation. In hiccup the MOSFET gates are turned
off for 110ms (hiccup timeout). Following the hiccup
timeout a soft-start is attempted. If OCP persists, hiccup
timeout will repeat. The IC will remain in hiccup mode until
load current is reduced below the programmed IOCP. In
order to program over-current protection use the following
equation:
(𝐼𝑂𝐶𝑃 × 𝑅𝐷𝑆) + 8𝑚𝑉
𝑅𝐿𝐼𝑀 =
𝐼𝐿𝐼𝑀
Where:
RLIM is resistor value for programming IOCP
IOCP is the over-current value to be programmed
RDS is the MOSFET rated on resistance
8mV is the OCP comparator offset
ILIM is the internal current that generates the necessary
OCP comparator threshold (use 45uA)
Note that ILIM has a positive temperature coefficient of
0.4%/°C. This is meant to roughly match and compensate
for positive temperature coefficient of the synchronous
FET. In order for this feature to be effective the
temperature rise of the IC should approximately match the
temperature rise of the FET.
SHORT-CIRCUIT PROTECTION (SCP)
If the output voltage drops below 60% of its programmed
value, the IC will enter hiccup mode. Hiccup will persist
until short-circuit is removed. SCP circuit becomes active
after PGOOD asserts high.
OVER-TEMPERATURE PROTECTION (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 figure 1 to
program the output voltage VOUT.
𝑉𝑂𝑈𝑇
𝑅1 = 𝑅2 × ( _0.6 − 1)
R2 recommended range is 2kΩ to 10kΩ.
PROGRAMMING THE SOFT-START
Place a capacitor CSS between the SS and GND 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:
10𝑢𝐴
𝐶𝑆𝑆 = 𝑇𝑆𝑆 × _{0.6𝑉}
FEED-FORWARD CAPACITOR (C_FF)
A feed-forward capacitor (CFF) may be necessary
depending on the Equivalent Series Resistance (ESR) of
COUT. If only ceramic output capacitors are used then a CFF
is necessary. Calculate CFF from:
1
𝐶𝐹𝐹 = _{2 × 𝜋 × 𝑅1 × 7 × 𝑓𝐿𝐶}
where:
R1 is the resistor that CFF is placed in parallel with
fLC is the frequency of the output filer double pole
fLC must be less than 15kHz when using ceramic COUT. If
necessary, increase COUT and/or L in order to meet this
constraint.
When using capacitors with higher ESR such as Panasonic
TPE series, a CFF is not required provided following
conditions are met:
1. The frequency of the output LC double pole fLC
should be less than 10kHz.
2. The frequency of ESR zero fZERO,ESR should be at
least five times larger than fLC.
Note that if fZERO,ESR is less than 5 x fLC, then it is
recommended to set the fLC at less than 2kHz. CFF is still
not required.
FEED-FORWARD RESISTOR (R_FF)
Poor PCB layout and/or extremely fast switching FETs can
cause switching noise at the output and may couple to the
FB pin via CFF. Excessive noise at FB will cause poor load
regulation. To solve this problem place a resistor RFF in
series with CFF. RFF value up to 2% of R1 is acceptable.
MAXIMUM ALLOWABLE VOLTAGE RIPPLE AT
FB PIN
Note that the steady-state voltage ripple at feedback pin
(VFB,RIPPLE) must not exceed 50mV in order for the controller
to function correctly. If VFB,RIPPLE is larger than 50mV then
COUT should be increased as necessary in order to keep the
VFB,RIPPLE below 50mV.
© 2015 Exar Corporation
11/14
Rev. 2.0.0