Rainbow-electronics MAX1703 Manual do Utilizador descarregar pdf (Página 11)

Shutdown

The MAX1703 shuts down to reduce quiescent current

to 1µA. During shutdown (ON = V

OUT

), the reference,

low-battery comparator, gain block, and all feedback

and control circuitry are off. The boost converter’s out-

put drops to one Schottky diode drop below the input.

Power-Good (POK) Comparator

The MAX1703 features an uncommitted POK compara-

tor. The internal POK comparator has an open-drain

output (POK) capable of sinking 1mA. When the input

(POKIN) rises above the 1.25V reference, the POK

open-drain output turns off. The POKIN input has 10mV

of hysteresis.

To provide a power-good signal, connect the POKIN

input to an external resistor-divider between OUT and

GND (Figure 5). Calculate the resistor values as follows:

R3 = R4(V

/ V

REF

- 1)

where V

is the desired input voltage trip threshold.

Since the input bias current into POKIN is less than

20nA, R4 can be a large value (such as 270kΩ or less)

without sacrificing accuracy. Connect the resistor volt-

age-divider as close to the IC as possible, within 0.2in.

(5mm) of POKIN.

Reference

The MAX1703 has an internal 1.250V, 1% bandgap ref-

erence. Connect a 0.22µF bypass capacitor to GND

within 0.2in. (5mm) of the REF pin. REF can source up

to 50µA of external load current.

Gain Block

The MAX1703 gain block can function as a second

comparator, or can be used to build a linear regulator

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 g

of the entire gain-block stage is 10mmho.

Figure 6 shows the gain block used in a linear-regulator

application. The output of an external P-channel pass

element is compared to the internal reference. The dif-

ference is amplified and used to drive the gate of the

pass element. Use a logic-level PFET, such as an

NDS336P (R

DS(ON)

= 270mΩ) from Fairchild. This con-

figuration allows ripple reduction at the output. If a

lower R

DS(ON)

PFET is used, then the linear regulator

output filter capacitance may need to be increased.

To use the gain block as a comparator, refer to the

Power-Good (POK) Comparator

section.

MAX1703

1-Cell to 3-Cell, High-Power (1.5A),

Low-Noise, Step-Up DC-DC Converter

______________________________________________________________________________________ 11

GNDPGND

POUT

OUT

LXP, LXN

MBR0520L

OUTPUT

100µF

2 x 220µF

4.7µH

POK

REF

CLK/SEL

AIN

POKIN

0.22µF

NOTE: HEAVY LINES INDICATE HIGH-CURRENT PATHS.

0.22µF

10Ω

MAX1703

SIGNAL GROUND

POWER GROUND

Figure 5. Adjustable Output (PWM Mode)

GNDPGND

POUT

OUT

LXP, LXN

100µF

MBR0520L

BOOST

OUTPUT

47µF

LINEAR

REGULATED

OUTPUT

20k

0.22µF

330µF

10Ω

0.22µF

4.7µH

POK

REF

CLK/SEL

AIN

POKIN

100k

MAX1703

SIGNAL GROUND

POWER GROUND

Figure 6. Using the Gain Block as a Linear Regulator

1 2 ... 6 7 8 9 10 11 12 13 14 15 16

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