XRP7662 Datasheet, PDF (14/19 Pages) Exar Corporation – 12A 300KHz Synchronous Step Down Regulator

XRP7 66 2
Pow er Blox ^TM
1 2 A 3 0 0 KHz Sy n ch r on ou s St ep Dow n Reg u lat or
where:
Æ©9_OUT = peak - to - peak output voltage ripple
I _PK-PK = peak - to - peak inductor ripple current
The total output ripple is a combination of the
ESR and the output capacitance value and can
be calculated as follows:
'VOUT
§I PP
¨
x1 D·²
¸
I PP
 xRESR ²
© ^{fs xCOUT} ¹
fs = Switching Frequency
D = Dut y Cycle
C_OUT = output capacitance value
I NPUT CAPACITOR SELECTION
The input capacitor should be selected for
ripple current rating, capacitance and voltage
rating. The input capacitor must meet the
ripple current requirement imposed by the
switching curren t. In continuous conduction
mode, the source current of the high - side
MOSFET is approximately a square wave of
duty cycle V _OUT/V _IN . More accurately, the
current wave form is trapezoidal, given a finite
turn - on and turn - off, switch transition slope.
Most of this current is supplied by the input
bypass capacitors. The RMS current handling
capability of the input capacitors is determined
at maximum output current and under the
assumption that the peak - to - peak inductor
ripple current is low, it is given by:
  I CIN (RMS) I OUT(MAX) x D 1 D
The worst case occurs when the duty cycle D
is 50% and gives an RMS current value equal
to Iout/2. Select input capacitors with
adequate ripple current rating to ensure
reliable operation.
The power dissipated in the input capacitor is:
P_CIN
=I
2
CIN(RMS)
‡_E5_SR(CIN)
This can become a significant part of power
losses in a converter and hurt the overall
energy transfer efficiency. The input voltage
ripple primarily depends on the input capacitor
ESR and capacitance. Ignoring the inductor
ripple current, the input voltage ripple can be
determined by:
'VIN
  _{I OUT( MAX)}
xRESR(CIN )
I

OUT
(
MAX
)
xVOUT
xVIN
VIN ² xfs xCIN
VOUT
The capacitor type suitable for the output
capacitors can also be used for the input
capacitors. However, exercise extra caution
when tantalum capacitors are used. Tantalu
m
capacitors are known for catastrophic failure
when exposed to surge current, and input
capacitors are prone to such surge current
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low impedance power sources. Although
tantalum capacitors have been successfully
employed at the input, it is generally not
recommended.
LOOP COMPENSATION D ESIGN
The open loop gain of the whole system can
be divided into the gain of the error amplifier,
PWM modulator, buck converter output stage,
and feedback resistor divider. In orde r to cross
over at the desired frequency cut - off (fco), the
gain of the error amplifier must compensate
for the attenuation caused by the rest of the
loop at this frequency. The goal of loop
compensation is to manipulate
loop frequency response such that i ts
crossover gain at 0db, results in a slope of
- 20db/decade.
The first step of compensation design is to
pick the loop crossover frequency. High
crossover frequency is desirable for fast
transient response, but often jeopardizes the
power supply stabilit y. Crossover frequency
should be higher than the ESR zero but less
than 1/5 of the switching frequency or
60kHz.
The ESR zero is contributed by the ESR
associated with the output capacitors and can
be determined by :
f _{Z ESR}
1
2S˜C_OUT R_ESR
The next step is to calculate the complex
conjugate poles contributed by the LC output
filter .
© 2012 Exar Corporation
14 / 19
Rev. 2. 2.0