MAX1605 Datasheet, PDF (7/12 Pages) Maxim Integrated Products – 30V Internal Switch LCD Bias Supply

30V Internal Switch LCD Bias Supply
V_CC
(2.4V TO 5.5V)
V_CC
MAX1605
LIM
V_CC
(2.4V TO 5.5V)
V_CC
NO CONNECTION
MAX1605
LIM
V_CC
(2.4V TO 5.5V)
V_CC
MAX1605
LIM
GND
IPEAK = 500mA
GND
IPEAK = 250mA
GND
IPEAK = 125mA
Figure 2. Setting the Peak Inductor Current Limit
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 10kΩ 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
30V. 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-voltage
batteries as well as high-voltage sources (0.8V to 30V)
because chip bias is provided by a logic supply (2.4V
to 5.5V), while the output power is sourced directly 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).
V_IN = 0.8V TO V_OUT
C_IN
10µF
L1
10µH
V_CC = 2.4V TO 5.5V _VCC
D1 V_OUT = 18V
LX
C1
0.1µF
MAX1605
R1
2.2MΩ
C_FF
C_OUT
10pF
1µF
ON
OFF
LIM
SHDN
FB
R2
165kΩ
GND
Figure 3. Typical Application Circuit
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