MAX1606 Maxim, MAX1606 Datasheet - Page 8

• Manufacturer Part Number: MAX1606
• Description: 28V Internal Switch LCD Bias Supply with True Shutdown
• Manufacturer: Maxim

28V Internal Switch LCD Bias Supply
with True Shutdown
Figure 3. Typical Application Circuit
Adjust the output voltage by connecting a voltage-
divider from the output (V
R2 between 10kΩ and 200kΩ. Calculate R1 with the fol-
lowing equation:
where V
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
The MAX1606 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
for 500mA, leave floating for 250mA, or connect to
GND for 125mA.
Pull SHDN low to enter shutdown. During shutdown the
supply current drops to 0.1µA, the output is discon-
nected from the input, and LX enters a high-impedance
state. The capacitance and load at the output deter-
mine the rate at which V
pulled as high as 6V, regardless of the input and output
voltages.
With the typical step-up converter circuit, the output
remains connected to the input through the inductor and
8
_______________________________________________________________________________________
V
V
BATT
CC
ON
FB
10µF
= 2.4V TO 5.5V
1µF
= 0.8V TO 5.5V
C2
C1
FB
).
= 1.25V and V
OFF
R1 = R2 [(V
Setting the Output Voltage (FB)
Current-Limit Select Pin (LIM)
SW
BATT
V
LIM
SHDN
CC
MAX1606
10µH
L1
OUT
OUT
OUT
OUT
GND
LX
FB
) to FB (Figure 3). Select
may range from V
/ V
decays. SHDN can be
Shutdown ( SHDN )
FB
D1
10pF
C
) – 1]
FF
1MΩ
75k
R1
R2
V
OUT
= 18V
BATT
C
1µF
OUT
CC
to
output rectifier, holding the output voltage to one diode
drop below V
allowing the output to draw power from the input. The
MAX1606 features true shutdown, which uses an internal
P-channel MOSFET to disconnect the output from the
input when the MAX1606 is shutdown. This eliminates
power drawn from the input during shutdown.
Separate voltage sources can supply the inductor
(V
ed by a logic supply (2.4V to 5.5V), this allows the out-
put power to be sourced directly from low-voltage
batteries (0.8V to 5.5V). Conversely, V
can also be supplied from one supply if it remains with-
in V
Smaller inductance values typically offer smaller physi-
cal size for a given series resistance or saturation cur-
rent. Circuits using larger inductance values may start
up at lower input voltages and exhibit less ripple, but
also provide reduced output power. This occurs when
the inductance is sufficiently large to prevent the maxi-
mum current limit from being reached before the maxi-
mum on-time expires. The inductor’s saturation current
rating should be greater than the peak switching cur-
rent. However, it is generally acceptable to bias the
inductor into saturation by as much as 20%, although
this will slightly reduce efficiency.
The peak LX current limit (I
application may be calculated from the following equa-
tion:
where t
mum voltage used to supply the inductor. The set cur-
rent limit must be greater than this calculated value.
Select the appropriate current limit by connecting LIM
to V
Limit Select Pin and Figure 2).
The high switching frequency of 500kHz requires a high-
speed rectifier. Schottky diodes, such as the Motorola
MBRS0530 or the Nihon EP05Q03L, are recommended.
To maintain high efficiency, the average current rating of
the Schottky diode should be greater than the peak
I
LX MAX
BATT
(
Separate/Same Power for V
CC
CC
’s operating limits (2.4V to 5.5V).
)
) and the IC (V
, GND, or leaving it unconnected (see Current-
≥
OFF(MIN)
V
OUT
V
BATT MIN
IN
×
I
when the converter is shutdown and
OUT MAX
= 0.8µs, and V
(
(
)
CC
Picking the Current Limit
). Since the chip bias is provid-
)
+
(
Design Procedure
V
OUT
LX(MAX)
Inductor Selection
−
BATT(MIN)
V
BATT MIN
Diode Selection
) required for the
2
BATT
(
×
BATT
L
)
)
×
is the mini-
and V
t
OFF MIN
and V
(
CC
)
CC