MAX1765(2000) Просмотр технического описания (PDF) - Maxim Integrated

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MAX1765
(Rev.:2000)

800mA, Low-Noise, Step-Up DC-DC Converter with 500mA Linear Regulator

Maxim Integrated 

800mA, Low-Noise, Step-Up DC-DC Converter

with 500mA Linear Regulator

INPUT

0.7V TO 5.5V

3.3µH

LX

TRACK

ON

OFF

OFF

ON

OUT

MAX1765

TRACK

ONL

ONB

ONA

REF

FB

POUT

INL

0.22µF

ILIM

OUTL

GND CLK/SEL FBL PGND

TRACK = LOW, VOUT = 5.0V

TRACK = HIGH, VOUT = 3.35V

33µF x 2

4.7Ω

0.68µF 150k

50k

4.7µF

VLDO = 2.85V

INPUT

MAX1765 LX

ONB

POUT

OUT

FB

OFF

ON

SHDN

OUTPUT

Figure 6. PFET Allows True Boost Shutdown

current limit increases and the output voltage rises. The

soft-start time constant is:

tRISE = RSSCSS

Figure 4. Dual-Output Power Supply

INPUT

ON

SHDN

LX

MAX1765

OFF

POUT

ONL

INL

ONA

OUTL

FBL

R3

R4

OUTPUT

Figure 5. LDO Enable Allows True Boost Shutdown

Placing a capacitor across the lower resistor of the cur-

rent-limiting resistive divider provides both features

simultaneously (Figure 9).

Package Selection

The MAX1765 is available in two packages, a 16-pin

QSOP and a thermally enhanced TSSOP-EP. The

QSOP is the less expensive of the two packages, and

requires a less complex layout design. This layout

allows the designer to route underneath the device. The

power dissipation for the QSOP is 0.7W.

The TSSOP-EP comes with an exposed metal pad that

is connected to the substrate of the IC. This increases

the power dissipation up to 1.5W for the TSSOP-EP. To

achieve maximum power capability, the exposed pad

of the TSSOP-EP should be reflowed to a pad with low

thermal resistance. For convenience, this pad can be

connected to AGND or PGND.

Inductor Selection

The MAX1765’s high switching frequency allows the

use of a small surface-mount inductor. For most appli-

cations, a 3.3µH inductor works well. The inductor

should have a saturation current rating exceeding the

N-channel switch current limit; however, it is accept-

able to bias the inductor current into saturation by as

much as 20% if a slight reduction in efficiency is

acceptable. Lower current-rated inductors may be

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