Datasheet

MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
22 ______________________________________________________________________________________
The transconductance error amplifier has a DC gain,
G
EA(DC)
= g
mEA
x R
O
, where g
mEA
is the error-amplifi-
er transconductance, which is equal to 110µS, R
O
is
the output resistance of the error amplifier, which is
30MΩ. A dominant pole is set by the compensation
capacitor (C
C
), the amplifier output resistance (R
O
),
and the compensation resistor (R
C
), and a zero is set
by the compensation resistor (R
C
) and the compensa-
tion capacitor (C
C
). There is an optional pole set by C
F
and R
C
to cancel the output-capacitor ESR zero if it
occurs near the crossover frequency (f
C
). Thus:
The crossover frequency, f
C
, should be much higher
than the power-modulator pole fp
MOD
. Also, f
C
should
be less than or equal to 1/5 the switching frequency.
Select a value for f
C
in the range:
At the crossover frequency, the total loop gain must
equal 1, and is expressed as:
For the case where f
zMOD
is greater than f
C
:
Then R
C
can be 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 fp
MOD
.
Calculate the value of C
C
as follows:
If f
zMOD
is less than 5 x f
C
, add a second capacitor,
C
F
, from COMP to GND. The value of C
F
is:
As the load current decreases, the modulator pole also
decreases; however, the modulator gain increases
accordingly and the crossover frequency remains the
same.
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
=
××
()
GG
f
f
MOD fc MOD dc
pMOD
C
() ( )
=×
GgR
EA fc mEA C()
=×
GG
V
V
EA fc MOD fc
FB
OUT
() ()
××=1
ff
f
pMOD C
S
<< ≤
5
f
CR
pEA
FC
=
××
1
2π
f
CR
zEA
CC
=
××
1
2π
f
CRR
pdEA
COC
=
×× +
1
2π ()
GAIN
(dB)
FREQUENCY
f
pMOD
f
zMOD
f
C
CLOSE LOOP
ERROR
AMP
0dB
FB
DIVIDER
POWER
MODULATOR
Figure 10. Simplified Gain Plot for the f
zMOD
> f
C
Case
GAIN
(dB)
FREQUENCY
f
pMOD
f
zMOD
f
C
CLOSE LOOP
ERROR
AMP
0dB
FB
DIVIDER
POWER
MODULATOR
Figure 11. Simplified Gain Plot for the f
zMOD
< f
C
Case