Datasheet
MAX1954A
Low-Cost, Current-Mode PWM Buck
Controller with Foldback Current Limit
______________________________________________________________________________________ 15
output filtering. For other types of capacitors, due to the
higher capacitance and ESR, the frequency of the zero
created by the capacitance and ESR is lower than the
desired closed-loop crossover frequency. Another
compensation capacitor should be added to cancel
this zero.
The basic regulator loop can be thought of as a power
modulator, output feedback divider, and an error ampli-
fier. The power modulator has DC gain set by g
mc
x
R
LOAD
, with a pole and zero pair set by R
LOAD
, the out-
put capacitor (C
OUT
) and its equivalent series resis-
tance (R
ESR
). Below are equations that define the
power modulator:
where R
LOAD
= V
OUT
/ I
OUT(MAX)
, and g
mc
= 1 / (A
CS
x
R
DS(ON)
), where A
CS
is the gain of the current-sense
amplifier and R
DS(ON)
is the on-resistance of the high-
side power MOSFET. A
CS
is 3.5. The frequencies at
which the pole and zero due to the power modulator
occur are determined as follows:
The feedback voltage-divider used has a gain of G
FB
=
V
FB
/ V
OUT
, where V
FB
is equal to 0.8V. The transcon-
ductance error amplifier has DC gain, G
EA(DC)
= g
m
x
R
O
. The amplifier output resistance (R
O
) is typically
10MΩ. The C
C
, R
O
, and the R
C
set a dominant pole.
The R
C
and the C
C
set a zero. There is an optional pole
set by C
F
and R
C
to cancel the output-capacitor ESR
zero if it occurs before crossover frequency (f
C
):
The f
C
should be much higher than the power modula-
tor pole f
PMOD
. Also, the crossover frequency should
be less than 1/8th of the switching frequency:
Therefore, the loop-gain equation at the crossover fre-
quency is:
When f
zMOD
is greater than f
C
:
then R
C
is calculated as:
where g
mEA
= 110µs.
The error-amplifier compensation zero formed by R
C
and C
C
should be set at the modulator pole f
pMOD
. C
C
is calculated by:
If f
zMOD
is less than 5 x f
C
, add a second compensa-
tion capacitor, C
f
, from COMP to GND to cancel the
ESR zero. C
f
is calculated by:
As the load current decreases, the modulator pole also
decreases. However, the modulator gain increases
accordingly and the crossover frequency remains
the same.
When f
zMOD
is less than f
C
, the power-modulator gain
at f
C
is:
GG
f
f
MOD fC MOD DC
pMOD
zMOD
() ( )
=×
C
Rf
f
C zMOD
=
××
1
2π
C
RfLC
RfLR
C
LOAD
S
OUT
LOAD
S
C
=
×××
+×
()
×
( )
R
V
gVG
C
OUT
mEA FB MOD fC
=
××
()
G g R and G g R
f
f
EA fC mEA C MOD fC mc LOAD
pMOD
C
() ()
=× =××
GG
V
V
EA fC MOD fC
FB
OUT
() ()
××=1
ff
f
pMOD C
S
<< <
8
f
CRR
f
CR
f
CR
pdEA
COC
zEA
CC
pEA
FC
=
××+
=
××
=
××
1
2
1
2
1
2
π
π
π
()
f
C
RfL
RfL
R
f
CR
pMOD
OUT
LOAD
S
LOAD
S
ESR
zMOD
OUT ESR
=
××
××
+×
+
⎛
⎝
⎜
⎞
⎠
⎟
=
××
1
2
1
2
π
π
Gg
RfL
RfL
MOD mc
LOAD
S
LOAD
S
=×
××
+×