Datasheet
using an external P-channel MOSFET pass device. The
gain-block output is a single-stage transconductance
amplifier that drives an open-drain N-channel MOSFET.
The transconductance (G
M
) of the entire gain-block
stage is 10mS. The internal gain block amplifies the dif-
ference between AIN and the internal 0.938V reference.
To provide a power-OK signal, connect the gain-block
input, AIN, to an external resistor-divider (Figure 3). The
input bias current into AIN is less than 30nA, allowing
large-value divider resistors without sacrificing accura-
cy. Connect the resistor voltage-divider as close to the
IC as possible, within 0.2in (5mm) of AIN. Choose an
R4 value of 270kΩ or less, then calculate R3 using:
R3 = R4((V
TRIP
/ V
AIN
) - 1)
where V
AIN
is 0.938V.
Figures 4 and 5 show the gain block used in a linear-
regulator application. The output of an external P-chan-
nel pass element is compared to an internal 0.938V
reference. The difference is amplified and drives the
gate of the pass element. Use a logic-level PFET, such
as Fairchild’s NDS336P (R
DS(ON)
= 270mΩ). When the
linear-regulator output voltage is in regulation, the
MOSFET will not be full on; thus, the on-resistance will
not be important. However, if the linear regulator is used
in dropout, the MOSFET on-resistance will determine
the dropout voltage (V
DROPOUT
= I
OUT
✕
R
DS(ON)
). If a
lower R
DS(ON)
PFET is used, increase the linear-regula-
tor output filter capacitance to maintain stability.
MAX1763
1.5A, Low-Noise, 1MHz, Step-Up
DC-DC Converter
______________________________________________________________________________________ 11
Figure 3. Using the Gain Block as a Power-OK Comparator
Table 3. On/Off Logic Control
ONA ONB MAX1763
00On
0 1 Off
10On
11On
MAX1763
POUT
AOAIN
R6
150k
R3
R4
POWER-OK
OUTPUT
TO V
IN
OR
V
OUT
GNDPGND
AO
POUT
OUT
LX
C1
47μF
BOOST
OUTPUT
C
OUT
47μF
LINEAR-
REGULATED
OUTPUT
R
G
20k
R5
4.7Ω
C2
1.0μF
0.22μF
V
IN
1.8V TO 5.5V
L1
1.5μH
FB
C4
220μF
REF
AIN
R3
R4
R2
30k
SIGNAL
GROUND
POWER
GROUND
R1
MAX1763
ISET
CLK/SEL
ONA
ONB
P
GNDPGND
POUT
OUT
LX
C4
220μF
R5
4.7Ω
C2
1μF
C3
0.22μF
V
IN
1.5μH
FB
C
OUT
47μF
C1
47μF
REF
AIN
R3
165k
R4
100k
MAX1763
ISET
CLK/SEL
AO
ONB
ONA
MBRO520L
3.3V
2.5V
R
G
20k
Figure 4. Using the Gain Block as a Linear Regulator from the
Boosted Output Voltage
Figure 5. Powering a Gain-Block Linear Regulator from the
Input Voltage