ISL8117A Datasheet, PDF (21/23 Pages) Intersil Corporation – Synchronous Step-Down PWM Controller

ISL8117A
The maximum capacitor value required to provide the full, rising
step, transient load current during the response time of the
inductor is shown in Equation 27:
C_OUT = ₂----_---_V----^_I--_N^L----^O-–---^--_V-^--^I-_O-^T---_-^R--_--^A_D----^N-_V---^-_O-²----_U----_T----_
(EQ. 27)
Where C_OUT is the output capacitor(s) required, L_O is the output
inductor, I_TRAN is the transient load current step, V_IN is the input
voltage, V_O is output voltage and DV_OUT is the drop in output
voltage allowed during the load transient.
High frequency capacitors initially supply the transient current
and slow the load rate of change seen by the bulk capacitors. The
bulk filter capacitor values are generally determined by the ESR
(Equivalent Series Resistance) and voltage rating requirements
as well as actual capacitance requirements.
The output voltage ripple is due to the inductor ripple current and
the ESR of the output capacitors as defined by Equation 28:
V_RIPPLE = I_LESR
(EQ. 28)
Where I_L is calculated in Equation 26.
High frequency decoupling capacitors should be placed as close
to the power pins of the load as physically possible. Be careful
not to add inductance in the circuit board wiring that could
cancel the usefulness of these low inductance components.
Consult with the manufacturer of the load circuitry for specific
decoupling requirements.
Use only specialized low-ESR capacitors intended for switching
regulator applications for the bulk capacitors. In most cases,
multiple small case electrolytic capacitors perform better than a
single large case capacitor.
In conclusion, the output capacitors must meet the following
criteria:
1. They must have sufficient bulk capacitance to sustain the
output voltage during a load transient while the output
inductor current is slewing to the value of the load transient.
2. The ESR must be sufficiently low to meet the desired output
voltage ripple due to the output inductor current.
The recommended output capacitor value for the ISL8117A is
between 100µF to 680µF, to meet the stability criteria with
external compensation. Use of aluminum electrolytic (POSCAP)
or tantalum type capacitors is recommended. Use of low ESR
ceramic capacitors is possible with loop analysis to ensure
stability.
Input Capacitor Selection
The important parameters for the input capacitor(s) are the
voltage rating and the RMS current rating. For reliable operation,
select input capacitors with voltage and current ratings above the
maximum input voltage and largest RMS current required by the
circuit. The capacitor voltage rating should be at least 1.25x
greater than the maximum input voltage and 1.5x is a
conservative guideline. The AC_RMS input current varies with the
load giving in Equation 29:
I_RMS = DC – DC²  I_O
(EQ. 29)
Where DC is duty cycle of the PWM.
The maximum RMS current supplied by the input capacitance
occurs at V_IN = 2 x V_OUT, DC = 50% as shown in Equation 30:
I_RMS = ¹²--  I_O
(EQ. 30)
Use a mix of input bypass capacitors to control the voltage ripple
across the MOSFETs. Use ceramic capacitors for the high
frequency decoupling and bulk capacitors to supply the RMS
current. Small ceramic capacitors can be placed very close to the
MOSFETs to suppress the voltage induced in the parasitic circuit
impedances.
Solid tantalum capacitors can be used, but caution must be
exercised with regard to the capacitor surge current rating. These
capacitors must be capable of handling the surge current at
power-up.
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FN8752.0
August 31, 2015