Datasheet

MAX1875/MAX1876
where t
ON(MIN)
is 100ns. The minimum input voltage is
limited by the switching frequency and minimum off-
time, which determine the maximum duty cycle
(D
MAX
= 1 - f
SW
t
OFF(MIN)
):
where V
DROP1
is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances. V
DROP2
is
the sum of the resistances in the charging path, includ-
ing high-side switch, inductor, and PC board resis-
tances.
Setting the Output Voltage
For 1V or greater output voltages, set the MAX1875/
MAX1876 output voltage by connecting a voltage-
divider from the output to FB_ to GND (Figure 6). Select
R_B (FB_ to GND resistor) to between 1k and 10k.
Calculate R_A (OUT_ to FB_ resistor) with the following
equation:
where V
SET
= 1V (see the Electrical Characteristics)
and V
OUT
can range from V
SET
to 18V.
For output voltages below 1V, set the MAX1875/
MAX1876 output voltage by connecting a voltage-
divider from the output to FB_ to REF (Figure 6). Select
R_C (FB to REF resistor) in the 1k to 10k range.
Calculate R_A with the following equation:
where V
SET
= 1V, V
REF
= 2V (see the Electrical
Characteristics), and V
OUT
can range from 0 to V
SET
.
RA RC
VV
VV
SET OUT
REF SET
__=
-
-
RA RB
V
V
OUT
SET
__=
-1
V
VV
ft
VV
IN MIN
OUT DROP
SW OFF MIN
DROP DROP()
()
=
+
+
1
21
1-
-
Dual 180° Out-of-Phase PWM Step-
Down Controllers with POR
12 ______________________________________________________________________________________
INDUCTOR CURRENT
I
LIMIT
I
LOAD
0 TIME
-I
PEAK
Figure 4. Valley Current-Limit Threshold Point
SYNC
SLAVE
OSC
SYNC
CK0
MASTER
V
L
4-PHASE SYSTEM2-PHASE SYSTEM
DH1
DH2
DH1
DH2
MASTER
SLAVE
180° PHASE SHIFT 90° PHASE SHIFT
DH1
DH2
DH1
DH2
MASTER
SLAVE
MAX1875
SYNC
SLAVE
OSC OSC
SYNC
CK0
MASTER
V
L
MAX1875 MAX1875
Figure 5. Synchronized Controllers