Rainbow Electronics MAX1765 - Manual (Page 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
R C
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
LX
OUT
REF
ILIM
4.7 Ω
0.68 µF 150k
V LDO = 2.85V
FB
33 µF x 2
3.3 µH
TRACK
ONL
ONB
ONA
OFF
ON
ON
OFF
TRACK
TRACK = LOW, V OUT = 5.0V
TRACK = HIGH, V OUT = 3.35V
0.22 µF
50k
4.7 µF
INL
Figure 5. LDO Enable Allows True Boost Shutdown
LX
POUT
INL
OUTL
OUTPUT
INPUT
MAX1765
ONL
ONA
SHDN
ON
OFF
FBL
R3
R4
Figure 6. PFET Allows True Boost Shutdown
LX
POUT
FB
OUT
OUTPUT
INPUT
MAX1765 SHDN
ONB
OFF
ON
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