Datasheet

MAX1927/MAX1928
For the MAX1928:
Resistors R1 and R2 are external to the MAX1927 (see
the Setting the Output Voltage section). I
OUT(MAX)
is the
maximum output current, R
CS
= 0.48V/A, and g
m
=
250µS for the MAX1927. See the Electrical Characteristics
table for MAX1928 g
m
values. Select the closest standard
C
C
value that gives an acceptable bandwidth.
3) Calculate the equivalent load impedance, R
L
, by:
4) Calculate the compensation resistance (R
C
) to can-
cel out the dominant pole created by the output
load and the output capacitance:
Solving for R
C
gives:
5) Calculate the high-frequency compensation pole to
cancel the zero created by the output capacitor’s ESR:
1
2
1
2ππ××
=
××RC RC
ESR OUT C f
R
RC
C
C
L OUT
C
=
×
1
2
1
2ππ××
=
××RC R C
L OUT C C
R
V
I
L
OUT
OUT MAX
=
()
C
V
IR
g
f
C
OUT
OUT MAX CS
m
C
=






×






×
()
×






()
11
2π
Low-Output-Voltage, 800mA, PWM Step-Down
DC-DC Converters
10 ______________________________________________________________________________________
BATT
COMP
GND
REF
PGND
LX
POK
FB
V
IN
2.6V TO 5.5V
C1
10µF
C2
10µF
C
C
1200pF
R
C
18kΩ
L1
CDRH4D18
4.7µH
V
OUT
1.8V AT 800mA
SHDN
PWM
MAX1928-18
C
f
22pF
C3
0.1µF
Figure 3. Applications Circuit for the MAX1928
BATT
COMP
REF
PGND
LX
POK
FB
V
IN
2.6V TO 5.5V
C1
10µF
C2
10µF
C
C
680pF
R
C
15kΩ
L1
CDRH4D18
4.7µH
V
OUT
1V AT 800mA
SHDN
MAX1927R
C
f
22pF
C3
0.1µF
R1
16.5kΩ
1%
R2
49.9kΩ
1%
GND
PWM
Figure 4. Applications Circuit for the MAX1927