Rainbow-electronics MAX1765 Manual de usuario descargar pdf (Pagina 15)

current limit increases and the output voltage rises. The

soft-start time constant is:

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

t =

RISE

SS SS

MAX1765

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

with 500mA Linear Regulator

Figure 4. Dual-Output Power Supply

MAX1765

INPUT

0.7V TO 5.5V

GND

CLK/SEL

FBL

PGND

OUTL

POUT

OUT

REF

ILIM

4.7Ω

0.68µF

150k

LDO

= 2.85V

33µF x 2

3.3µH

TRACK

ONL

ONB

ONA

OFF

TRACK

TRACK = LOW, V

OUT

= 5.0V

TRACK = HIGH, V

OUT

= 3.35V

0.22µF

50k

4.7µF

INL

Figure 5. LDO Enable Allows True Boost Shutdown

POUT

INL

OUTL

OUTPUT

INPUT

MAX1765

ONL

ONA

SHDN

OFF

FBL

Figure 6. PFET Allows True Boost Shutdown

POUT

OUT

OUTPUT

INPUT

MAX1765

SHDN

ONB

OFF

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