Datasheet

MAX1858
Design Procedure
Effective Input Voltage Range
Although, the MAX1858 controller can operate from
input supplies ranging from 4.75V to 23V, the input volt-
age range can be effectively limited by the MAX1858’s
duty-cycle limitations. The maximum input voltage is
limited by the minimum on-time (t
ON(MIN)
):
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 recti-
fier, inductor, and PC board resistances. V
DROP2
is the
sum of the resistances in the charging path, including
high-side switch, inductor, and PC board resistances.
Setting the Output Voltage
For 1V or greater output voltages, set the MAX1858 out-
put voltage by connecting a voltage-divider from the
output to FB_ to GND (Figure 6). Select R_B (FB_ to
GND resistor) to between 1kΩ and 10kΩ. Calculate
R_A (OUT_ to FB_ resistor) with the following equation:
where V
SET
= 1.00V (see the Electrical Characteristics)
and V
OUT
can range from V
SET
to 18V.
RA RB
V
V
OUT
SET
__=














-1
V
VV
ft
VV
IN MIN
OUT DROP
SW OFF MIN
DROP DROP()
()
=
+








+
1
21
1-
-
V
V
tf
IN MAX
OUT
ON MIN SW
()
()
≤
Dual 180° Out-of-Phase PWM Step-Down
Controller with Power Sequencing and 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
MAX1858
SYNC
SLAVE
OSC OSC
SYNC
CK0
MASTER
V
L
MAX1858 MAX1858
Figure 5. Synchronized Controllers