Shdn – Rainbow Electronics MAX758A User Manual
Page 7
MAX1605
28V Internal Switch LCD Bias Supply
in SOT23
_______________________________________________________________________________________
7
to 500kHz and depends upon the load and input volt-
age. The peak current limit of the internal N-channel
MOSFET is pin selectable and may be set at 125mA,
250mA, or 500mA (Figure 2).
Setting the Output Voltage (FB)
Adjust the output voltage by connecting a voltage-
divider from the output (V
OUT
) to FB (Figure 3). Select
R2 between 10k
Ω to 200kΩ. Calculate R1 with the fol-
lowing equation:
R1 = R2 [(V
OUT
/ V
FB
) – 1]
where V
FB
= 1.25V and V
OUT
may range from V
IN
to
28V. The input bias current of FB has a maximum value
of 100nA, which allows large-value resistors to be used.
For less than 1% error, the current through R2 should
be greater than 100 times the feedback input bias cur-
rent (I
FB
).
Current Limit Select Pin (LIM)
The MAX1605 allows a selectable inductor current limit
of 125mA, 250mA, or 500mA (Figure 2). This allows
flexibility in designing for higher current applications or
for smaller, compact designs. The lower current limit
allows the use of a physically smaller inductor in space-
sensitive, low-power applications. Connect LIM to V
CC
for 500mA, leave floating for 250mA, or connect to
GND for 125mA.
Shutdown (
SHDN
)
Pull SHDN low to enter shutdown. During shutdown, the
supply current drops to 0.1µA and LX enters a high-
impedance state. However, the output remains con-
nected to the input through the inductor and output
rectifier, holding the output voltage to one diode drop
below V
IN
when the MAX1605 is shut down. The capac-
itance and load at OUT determine the rate at which
V
OUT
decays. SHDN can be pulled as high as 6V,
regardless of the input and output voltages.
Separate/Same Power for L1 and V
CC
Separate voltage sources can supply the inductor (V
IN
)
and the IC (V
CC
). This allows operation from low-volt-
age batteries as well as high-voltage sources (0.8V to
28V) because chip bias is provided by a logic supply
(2.4V to 5.5V) while the output power is sourced direct-
ly from the battery to L1. Conversely, V
IN
and V
CC
can
also be supplied from one supply if it remains within
V
CC
’s operating limits (+2.4V to +5.5V).
GND
LIM
GND
V
CC
V
CC
LIM
GND
V
CC
LIM
NO CONNECTION
V
CC
(2.4V TO 5.5V)
V
CC
(2.4V TO 5.5V)
V
CC
(2.4V TO 5.5V)
IPEAK = 500mA
IPEAK = 250mA
IPEAK = 125mA
MAX1605
MAX1605
MAX1605
Figure 2. Setting the Peak Inductor Current Limit
V
IN
= 0.8V TO V
OUT
V
CC
= 2.4V TO 5.5V
V
OUT
= 18V
ON
OFF
SHDN
V
CC
LIM
GND
LX
FB
MAX1605
L1
10
µH
R2
165k
R1
2.2M
Ω
D1
C1
0.1
µF
C
FF
10pF
C
OUT
1
µF
C
IN
10
µF
Figure 3. Typical Application Circuit