SP6134H Datasheet, PDF (9/15 Pages) Sipex Corporation – High Voltage, 600 KHz Synchronous PWM Controller

SP6134H
High Voltage, 600kHz Synchronous PWM Step Down
Controller
exceed the tolerance limits expected on the
output voltage. During an output load
transient, the output capacitor must supply all
the additional current demanded by the load
until the SP6134HCU adjusts the inductor
current to the new value.
Therefore the capacitance must be large
enough so that the output voltage is help up
while the inductor current ramps up or down
to the value corresponding to the new load
current. Additionally, the ESR in the output
capacitor causes a step in the output voltage
equal to the current. Because of the fast
transient response and inherent 100% and 0%
duty cycle capability provided by the
SP6134HCU when exposed to output load
transient, the output capacitor is typically
chosen for ESR, not for capacitance value.
The output capacitor’s ESR, combined with the
inductor ripple current, is typically the main
contributor to output voltage ripple. The
maximum allowable ESR required to maintain
a specified output voltage ripple can be
calculated by:
R_ESR
≤
ΔV_OUT
I _PP
ΔV_OUT = Peak to Peak Output Voltage Ripple
I_PP = 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:
( ) _{( ) ΔVOUT =}
⎜⎜⎝⎛
I
PP ^{1 − D}
C_OUT F_S
⎟⎟⎠⎞^{2 +}
I _PP R_ESR
2
Where:
FS = Switching Frequency
D = Duty Cycle
C_OUT = Output Capacitance Value
INPUT 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 current. In continuous conduction
mode, the source current of the high-side
MOSFET is approximately a square wave of
duty cycle VOUT/VIN. Most of this current is
supplied by the input bypass capacitors. The
RMS value of input capacitor current is
determined at the 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)} D 1 − D
The worse case occurs when the duty cycle D
is 50% and gives an RMS current value equal
to I_OUT/2.
Select input capacitors with adequate ripple
current rating to ensure reliable operation. The
power dissipated in the input capacitor is:
P_CIN = I R ²CIN (rms) _{ESR(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:
( ) _ΔVIN
= I _{OUT (max)}R_{ESR(CIN )} +
I_{OUT (max)}V_OUT V_IN − V_OUT
F_S C_INV_IN ²
The capacitor type suitable for the output
capacitors can also be used for the input
capacitors. However, exercise extra caution
when tantalum capacitors are considered.
Tantalum capacitors are known for
catastrophic failure when exposed to surge
current, and input capacitors are prone to
such surge current when power supplies are
connected “live” to low impedance power
sources.
© 2008 Exar Corporation
9/15
Rev. 2.0.0