xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
MAX1864/MAX1865
D1
CENTRAL CMPSH-3
INPUT
9V TO 18V
NL, NH: INTERNATIONAL RECTIFIER
IRF7303
Q1: TIP30
Q2: 2N3905
C
IN
470µF
IN
C1
1µF
DH
C
BST
0.1µF
VL
C2
1µF
R
POK
100kΩ
POK
LX
R
DL
10Ω
ILIM
DL
N
L
BST
R
DH
10Ω
N
H
T1
1
C
OUT
470µF
V
OUT
= 3.3V
1A
MAX1864
OUT
GND
FB
C
BE2
2200pF
R
BE2
220Ω
R1
10kΩ
R2
10kΩ
C3
10µF
Q1
V
OUT2
= 2.5V
300mA
C4
10µF
T1
R
COMP
47kΩ
COMP
C
COMP
8.2nF
B2
FB2
C
BE3
4700pF
R
BE3
220Ω
R3
30kΩ
C6
10µF
D2
NIHON EP05Q03L
C5
470µF
B3
Q2
FB3
R4
10kΩ
C7
10µF
V
OUT3
= 5.0V
100mA
Figure 1. Standard MAX1864 Application Circuit
the voltage across the load. Under overload conditions,
when the inductor current exceeds the selected cur-
rent-limit (see
Current Limit),
the high-side MOSFET is
not turned on at the rising edge of the clock and the
low-side MOSFET remains on to let the inductor current
ramp down.
The MAX1864/MAX1865 operate in a forced-PWM
mode, so even under light loads the controller main-
tains a constant switching frequency to minimize cross-
regulation errors in applications that use a transformer.
The low-side gate-drive waveform is the complement of
the high-side gate-drive waveform, which causes the
inductor current to reverse under light loads.
Current-Sense Amplifier
The MAX1864/MAX1865s’ current-sense circuit ampli-
fies (A
V
= 5) the current-sense voltage generated by
the high-side MOSFET’s on-resistance (R
DS(ON)
✕
I
INDUCTOR
). This amplified current-sense signal and
the internal slope compensation signal are summed
together (V
SUM
) and fed into the PWM comparator’s
inverting input. The PWM comparator turns-off the high-
side MOSFET when V
SUM
exceeds the integrated feed-
back voltage (V
COMP)
. Place the high-side MOSFET no
further than 5mm from the controller, and connect IN
and LX to the MOSFET using Kelvin sense connections
to guarantee current-sense accuracy and improve
stability.
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11
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