Datasheet
MAX1960/MAX1961/MAX1962
A small capacitor C
F
, can also be added from COMP to
GND to provide high-frequency decoupling. C
F
will add
another high-frequency pole (f
PHF
) to the error-amplifier
response. This pole should be greater than 100 times
the error-amplifier zero frequency to have negligible
impact on the phase margin. This pole should also be
less than half the switching frequency for effective
decoupling:
100f
ZEA
< f
PHF
< 0.5f
S
Select a value for f
PHF
in the range given above, then
solve for C
F
using the following equation:
Below is a numerical example to calculate compensa-
tion values:
V
IN
= 3.3V
V
RAMP
= 0.85V
V
OUT
= 1.8V
V
FB
= 0.8V
I
OUT(max)
= 15A
C
O
= 2 x 680µF = 1360µF
ESR = 0.008Ω / 2 = 0.004Ω
L
O
= 0.22µH
g
mEA
= 2mS
f
S
= 1MHz
Choose the crossover frequency (f
C
) in the range f
ZESR
< f
C
< f
S
/5:
29.3kHz < f
C
< 200kHz
Select f
C
= 100kHz, this meets the criteria above, and the
bandwidth is high enough for good transient response.
The power modulator gain at f
C
is:
Choose R
1
= 8.06kΩ, then R
2
= 10kΩ (see the
Setting
the Output Voltage
section):
Select C
C
= 8200pF (nearest standard capacitor
value).
Select f
PHF
in the range 100f
ZEA
< f
PHF
< 0.5f
S
.
184kHz < f
PHF
< 500kHz
Select f
PHF
=
250kHz, then solve for C
F
:
Select the nearest standard capacitor value C
F
= 56pF.
Summary of feedback divider and compensation com-
ponents:
R
1
= 8.06kΩ
R
2
= 10kΩ
R
C
= 11kΩ
C
C
= 8200pF
C
F
= 56pF
Power MOSFET Selection
When selecting a MOSFET, essential parameters
include:
(1) Total gate charge (Q
G
)
(2) Reverse transfer capacitance (C
RSS
)
(3) On-resistance (R
DS(ON)
)
C
R f k kHz
pF
F
C PHF
=
××
=
××
=
1
2
1
2 11 250
58
ππ Ω
C
Rf k
pF
C
C PMOD
=
××
=
××
=
5
2
5
2 11 9201
7863
ππ Ω
V
gVG
k
C
OUT
mEA FB MOD f
C
=
××
=
××
=
.
. . .
()
18
0 002 0 8 0 102
11 Ω
G
V
V
f
ff
kk
MOD fc
IN
RAMP
PMOD
ZESR C
()
()
.
()
.
.
=×
×
=×
×
=
2
2
3
085
9201
29 3 100
0 102
ΩΩ
f
CR
kHz
ZESR
O ESR
=
××
=
×××
=
−
1
2
1
2 1360 10 0 004
29 3
6
π
π
.
.
f
LC
kHz
PMOD
OO
=
××
=
××××
=
−−
1
2
1
2 0 22 10 1360 10
9 201
66
π
π
.
.
C
Rf
F
C PHF
=
××
1
2π
C
Rf
C
C PMOD
=
××
5
2π
2.35V to 5.5V, 0.5% Accurate, 1MHz PWM
Step-Down Controllers with Voltage Margining
18 ______________________________________________________________________________________