MIC25400 Datasheet, PDF (15/27 Pages) Micrel Semiconductor – 2A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
MIC25400
Figure 9. Overcurrent waveform
Figure 9 shows the low-side MOSFET current waveform.
Peak current is measured after a small delay. The
equations used to calculate the current limit resistor
value are shown below:
I_PK
= I_OUT
+ ^IPP
2
I_OC
= I_PK
−
V_OUT ⋅ T_DLY
L
R _CS
=
I_OC
⋅ RDS_ON
I_CS
Where:
I_OC is the current limit set point
L = inductor value
T_DLY = Current limit blanking time ~ 100ns
I_CS is the overcurrent pin sense current (200µA nominal)
RDS_ON is the on resistance of the low-side MOSFET
Snubber
A snubber is used to damp out high frequency ringing
caused by parasitic inductance and capacitance in the
buck converter circuit. Figure 10 shows a simplified
schematic of one of the buck converter phases. Stray
capacitance consists mostly of the two MOSFET’s
output capacitance (C_OSS). The stray inductance is
mostly package and etch inductance. The arrows show
the resonant current path when the high-side MOSFET
turns on. This ringing causes stress on the
semiconductors in the circuit as well as increased EMI.
Figure 10. Output Parasitics
One method of reducing the ringing is to use a resistor to
lower the Q of the resonant circuit. The circuit in Figure
11 shows an RC network connected between the switch
node and ground. Capacitor C_S is used to block DC and
minimize the power dissipation in the resistor. This
capacitor value should be between five and ten times the
parasitic capacitance of the MOSFET C_OSS. A capacitor
that is too small will have high impedance and prevent
the resistor from damping the ringing. A capacitor that is
too large causes unnecessary power dissipation in the
resistor, which lowers efficiency.
The snubber components should be placed as close as
possible to the low-side MOSFET and/or external
schottky diode since in contributes to most of the stray
capacitance. Placing the snubber too far from the FET or
using an etch that is too long or too thin adds inductance
to the snubber and diminishes its effectiveness.
January 2011
Figure 11. Snubber Circuit
15
M9999-020111-C