Rainbow Electronics MAX1541 - Manual (Page 32)

MAX1540/MAX1541
Setting V OUT with a Resistive Voltage-Divider at FB_
The output voltage can be adjusted from 0.7V to 5.5V
using a resistive voltage-divider (Figure 11). The
MAX1540 regulates FB1 and FB2 to a fixed 0.7V refer-
ence voltage. The MAX1541 regulates FB1 to the volt-
age set at REFIN1 and regulates FB2 to a fixed 0.7V
reference voltage. This makes the main MAX1541 con-
troller (OUT1) ideal for memory applications where the
termination supply must track the supply voltage. The
adjusted output voltage is:
where V
FB_
= 0.7V for the MAX1540, and V
FB1
=
V REFIN1 and V FB2 = 0.7V for the MAX1541.
Dynamic Output Voltages (MAX1541 OUT1 Only)
The MAX1541 regulates FB1 to the voltage set at
REFIN1. By changing the voltage at REFIN1, the
MAX1541 can be used in applications that require
dynamic output-voltage changes between two set
points. Figure 12 shows a dynamically adjustable resis-
tive voltage-divider network at REFIN1. Using the GATE
signal and open-drain output (OD), a resistor can be
switched in and out of the REFIN1 resistor-divider,
changing the voltage at REFIN1. A logic high on GATE
turns on the internal N-channel MOSFET, forcing OD to
a low-impedance state. A logic low on GATE disables
the N-channel MOSFET, so OD is high impedance. The
two output voltages (FB1 = OUT1) are determined by
the following equations:
The main MAX1541 controller (OUT1) automatically
enters forced-PWM operation on the rising and falling
edges of GATE, and remains in forced-PWM mode for a
minimum time selected by FBLANK (Table 6). Forced-
PWM operation is required to ensure fast, accurate
negative voltage transitions when REFIN1 is lowered.
Since forced-PWM operation disables the zero-crossing
comparator, the inductor current may reverse under
V V
V V
OUT1(LOW) REF
OUT1(HIGH) REF
R9
R8+R9
(R9+R10)
R8+(R9+R10)
=




=






V V
OUT_ FB_
C
D
R
R
= +

 

1
Dual Step-Down Controllers with Saturation
Protection, Dynamic Output, and Linear Regulator
32 ______________________________________________________________________________________
Figure 10. MAX1540/MAX1541 Dual Mode Feedback Decoder
MAX1540
MAX1541
TO ERROR
AMPLIFIER
FB_
REF
(2.0V)
FIXED OUTPUT
FB = GND
FIXED OUTPUT
FB = V
CC
OUT_
9R
R
Table 8. Output Voltage Configuration
OUT1 OUT2
MAX1540 MAX1541 MAX1540 MAX1541
FB_ = V CC
Fixed
1.2V
Not
allowed
Fixed
1.5V
Fixed
1.8V
FB_ = GND
Fixed
1.8V
Not
allowed
Fixed
2.5V
Fixed
2.5V
FB_ = OUT_
or adjustable
0.7V V REFIN1 0.7V 0.7V