© 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN23SV60 • Rev. 1.10 16
FAN23SV60 — 10 A Synchronous Buck Regulator
Inductor Selection
The inductor is typically selected based on the ripple
current (IL), which is approximately 25% to 45% of the
maximum DC load. The inductor current rating should
be selected such that the saturation and heating current
ratings exceed the intended currents encountered in the
application over the expected temperature range of
operation. Regulators that require fast transient
response use smaller inductance and higher current
ripple; while regulators that require higher efficiency
keep ripple current on the low side.
The inductor value is given by:
For example: for 19 V VIN, 1.2 V VOUT, 10 A load, 30%
IL, and 500 kHz fSW; L is 720 nH.
Input Capacitor Selection
Input capacitor CIN is selected based on voltage rating,
RMS current ICIN(RMS) rating, and capacitance. For
capacitors having DC voltage bias derating, such as
ceramic capacitors, higher rating is strongly
recommended. RMS current rating is given by:
where ILOAD-MAX is the maximum load current and D is
the duty cycle VOUT/VIN. The maximum ICIN(RMS) occurs
at 50% duty cycle.
The capacitance is given by:
where VIN is the input voltage ripple, normally 1% of
VIN.
For example; for VIN=19 V, VIN=120 mV, VOUT=1.2 V,
10 A load, and fSW=500 kHz; CIN is 9.8 F and ICIN(RMS) is
2.4 ARMS. Select a minimum of two 10 F 25 V-rated
ceramic capacitors with X7R or similar dielectric,
recognizing that the capacitor DC bias characteristic
indicates that the capacitance value falls approximately
60% at VIN=19 V. Also, each 10 µF can carry over
3 ARMS in the frequency range from 100 kHz to 1 MHz,
exceeding the input capacitor current rating
requirements. An additional 1 µF capacitor may be
needed to suppress noise generated by high frequency
switching transitions
Output Capacitor Selection
Output capacitor COUT is also selected based on voltage
rating, RMS current ICIN (RMS) rating, and
capacitance. For capacitors having DC voltage bias
derating, such as ceramic capacitors, higher rating is
highly recommended.
When calculating COUT, usually the dominant
requirement is the current load step transient. If the
unloading transient requirement (IOUT transitioning from
HIGH to LOW), is satisfied, then the load transient (IOUT
transitioning LOW to HIGH), is also usually satisfied.
The unloading C OUT calculation, assuming COUT has
negligible parasitic resistance and inductance in the
circuit path, is given by:
where IMAX and IMIN are maximum and minimum load
steps, respectively and VOUT is the voltage
overshoot, usually specified at 3 to 5%.
For example: for VI=19 V, VOUT=1.2 V, 6A IMAX, 2 A IMIN,
fSW=500 kHz, LOUT=720 nH, and 3% VOUT deviation of
36 mV; the COUT value is calculated to be 263µF. This
capacitor requirement can be satisfied using six 47 µF,
6.3 V-rated X5R ceramic capacitors. This calculation
applies for load current slew rates that are faster than
the inductor current slew rate, which can be defined as
VOUT/L during the load current removal. For reduced-
load-current slew rates and/or reduced transient
requirements, the output capacitor value may be
reduced and comprised of low-cost 22 µF capacitors.
Setting the Current Limit
Current limit is implemented by sensing the inductor
valley current across the LS MOSFET VDS during the LS
on-time. The current limit comparator prevents a new
on-time from being started until the valley current is less
than the current limit.
The set point is configured by connecting a resistor from
the ILIM pin to the SW pin. A trimmed current is output
onto the ILIM pin, which creates a voltage across the
resistor. When the voltage on ILIM goes negative, an
over-current condition is detected.
RILIM is calculated by:
where KILIM is the current source scale factor, and
IVALLEY is the inductor valley current when the current
limit threshold is reached. The factor 1.04 accounts
for the temperature offset of the LS MOSFET
compared to the control circuit.
With the constant on-time architecture, HS is always
turned on for a fixed on-time; this determines the peak-
to-peak inductor current.
Current ripple I is given by:
From the equation above, the worst-case ripple occurs
during an output short circuit (where VOUT is 0 V). This
should be taken into account when selecting the current
limit set point.
The FAN23SV60 uses valley-current sensing, the
current limit (IILIM) set point is the valley (IVALLEY).
The valley current level for calculating RILIM is given by: