Exceeds the integrated feed- back voltage (v – Rainbow Electronics MAX1865 User Manual
Page 11
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________
11
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.
BST
C1
1
µF
D1
CENTRAL CMPSH-3
C2
1
µF
N
H
N
L
C
BST
0.1
µF
R
POK
100k
Ω
C
COMP
C
OUT
470
µF
C
IN
470
µF
DH
LX
T1
V
OUT
= 3.3V
1A
INPUT
9V TO 18V
DL
OUT
GND
IN
VL
ILIM
POK
COMP
MAX1864
FB
8.2nF
R
COMP
47k
Ω
R
DH
10
Ω
1
R
DL
10
Ω
R
BE2
220
Ω
C3
10
µF
Q1
R1
10k
Ω
V
OUT2
= 2.5V
300mA
C4
10
µF
T1
R2
10k
Ω
R
BE3
220
Ω
Q2
C6
10
µF
R3
30k
Ω
R4
10k
Ω
C7
10
µF
V
OUT3
= 5.0V
100mA
C5
470
µF
D2
NIHON EP05Q03L
B2
FB2
B3
FB3
C
BE3
4700pF
C
BE2
2200pF
NL, NH: INTERNATIONAL RECTIFIER
IRF7303
Q1: TIP30
Q2: 2N3905
Figure 1. Standard MAX1864 Application Circuit