3
AN1288.1
October 30, 2008
With Infineon’s BSC030N03LS as the low-side MOSFET and
R
BSOC
of 1.74k
Ω
. The overcurrent trip point on the evaluation
board has been set to 21A for 12V
BIAS
(17A for 5V
BIAS
).
Feedback Compensator
Type-III network is recommended for compensating the
feedback loop. Figure 1 shows Type-III compensation
configuration for ISL8105B.
With the inductor and output capacitor selected as described
in the previous sections, the poles and zero of the power
stage can be summarized in Equation 15:
1. With a value of 11.8k
Ω
for R
1
, select R
4
for the target
output voltage of 1.8V using Equation 16:
2. With the desired feedback loop bandwidth of 30kHz, R
2
can be calculated using Equation 17:
3. Select C
1
such that F
Z1
is located at 1.5kHz (~50% of
F
LC
):
4. Select C
2
such that F
P1
is located at F
ESR
:
5. Select R
3
such that F
Z2
is located at F
LC
:
A more detailed explanation of designing compensation
networks for buck converters with voltage mode control can
be found in TB417 entitled “Designing Stable Compensation
Networks for Single Phase Voltage Mode Buck Regulators”.
Evaluation Board Performance
Figure 2 shows a photograph of the ISL8105BEVAL1Z.
Power and Load Connections
Terminals J1 and J2 are connected to the input of the power
stage. The IC bias supply and the converter input supply can
be together through pin 2 and 3 of the Jumper J7 to provide
single rail supply application. When using separate supplies,
provide the IC bias voltage to terminal J2 with pin 2 and pin 1
of J7 connected together. The load can be connected to
terminal J4 and J5. TP6 and TP3 can be used for DMM to
measure output voltage. The scope probe terminal (TPV01)
can be used to monitor V
OUT
with an oscilloscope. The push
switch, SW1, can be used to disable the controller.
FIGURE 1. VOLTAGE-MODE BUCK CONVERTER
COMPENSATION DESIGN
-
+
E/A
VREF
COMP
C
1
R
2
R
1
FB
C
2
R
3
C
3
L
C
V
IN
PWM
CIRCUIT
HALF-BRIDGE
DRIVE
OSCILLATOR
ESR
EXTERNAL CIRCUIT
ISL8105B
V
OUT
V
OSC
DCR
TGATE
LX
BGATE
R
4
F
0
1
2
π
×
L
C
×
×
-------------------------------------
3.7kHz
=
=
F
ESR
1
2
π
×
C
ESR
×
×
-------------------------------------------
33.9kHz
=
=
(EQ. 15)
R
4
R
1
V
ref
V
OUT
V
ref
–
---------------------------------
×
=
5.9k
Ω
=
(EQ. 16)
R
2
V
OSC
R
1
F
0
⋅
⋅
d
max
V
IN
F
LC
⋅
⋅
--------------------------------------------
=
12k
Ω
=
(EQ. 17)
C
1
1
2
π
R
2
1.5
3
×
10
⋅
⋅
--------------------------------------------
=
10nF
≈
(EQ. 18)
C
2
C
1
2
π
R
2
C
1
F
ESR
⋅
⋅
⋅
1
–
------------------------------------------------------------
=
390pF
≈
(EQ. 19)
R
3
R
1
150
3
×
10
F
LC
----------------------
1
–
--------------------------------
301
Ω
≈
=
C
3
1
2
π
R
3
150
3
×
10
⋅
⋅
----------------------------------------------
3.3nF
≈
=
(EQ. 20)
FIGURE 2. ISL8105BEVAL1Z
Application Note 1288
