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.

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 270k

Ω or less, then calculate R3 using:

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

Ω). 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

tor output filter capacitance to maintain stability.

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

00

On

0

1

Off

10

On

11

On

MAX1763

POUT

AO

AIN

R6

150k

R3

R4

POWER-OK

OUTPUT

GND

PGND

AO

POUT

OUT

LX

C1

47

μF

BOOST

OUTPUT

47

μF

LINEAR-

REGULATED

OUTPUT

20k

R5

4.7

Ω

C2

1.0

μF

0.22

μF

1.8V TO 5.5V

L1

1.5

μH

FB

C4

220

μF

REF

AIN

R3

R4

R2

30k

SIGNAL

GROUND

POWER

GROUND

R1

MAX1763

ISET

CLK/SEL

ONA

ONB

P

GND

PGND

POUT

OUT

LX

C4

220

μF

R5

4.7

Ω

C2

1

μF

C3

0.22

μF

1.5

μH

FB

47

μF

C1

47

μF

REF

AIN

R3

165k

R4

100k

MAX1763

ISET

CLK/SEL

AO

ONB

ONA

MBRO520L

3.3V

2.5V

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