3
AN1482.0
June 30, 2009
Load Transient Circuit Set-up
1. Select a SOIC8 N-Channel MOSFET with VDSS
breakdown > 20V.
2. Install the load transient circuit as indicated on the
schematic. Refer to Figure 4 for detail.
3. R27, R22 and R25 are 10k
Ω
resistors for discharging the
MOSFET gates.
4. R26, R23 and R24 are current sensing resistors to
monitor the load step. For accurate measurement, please
use 5% tolerance sensing resistor or better. To alleviate
thermal stress, use 0.1
Ω
or smaller resistance. The
resistance of the sensing resistors sets the current scale
on the oscilloscope.
5. Apply pulse square waveform across R27, R22 or R25.
The duty cycle of the pulse waveform should be small
(<5%) to limit thermal stress on current sensing resistor
and the MOSFETs (M8, M6 or M7)
6. The amplitude of the clock sets the current step
amplitude. Adjust the clock amplitude and slew rate to set
the current step and slew rate.
7. Monitor overshoot and undershoot at corresponding
output.
A
V
+
_
+ VIN -
LO
A
D
LO
A
D
LO
A
D
A
+
_
A
A
V
+ -
V
V
+ -
+
_
+
_
+
_
Vo1
Vo2
Vo3
Io1
Io2
Io3
FIGURE 2. PROPER TEST SET-UP
FIGURE 3. PROPER PROBE SET-UP TO MEASURE OUTPUT
RIPPLE AND PHASE NODE RINGING
OUTPUT CAP
OR MOSFET
OUTPUT CAP
OR MOSFET
OUTPUT CAP
OR MOSFET
Application Note 1482
