MAX856CSA-T Maxim Integrated Products, MAX856CSA-T Datasheet - Page 8

• Manufacturer Part Number: MAX856CSA-T
• Description: DC/DC Switching Converters 3.3/5/AdjV Step-Up DC/DC Converter
• Manufacturer: Maxim Integrated Products

The 3/5 input is internally diode clamped
OUT, and should not be connected to signals outside
this range. The SHDN input and LBO output (open-
drain) are not clamped to V+ and can be pulled as high
as 7V regardless of the voltage at OUT
control inputs (3/5, LBI, or SHDN) floating.
For the MAX856/MAX858,you can select a 3.3V or 5V
output voltage under logic control, or by tying 3/
GND or OUT. Efficiency is typically better than 80%
over a 2mA to 100mA (MAX856/MAX857) load range.
The device is internally bootstrapped, with power
derived from the output voltage (via OUT). When the
output is in 5V mode, the higher internal supply voltage
results in lower switch-transistor on-resistance, slightly
greater output power, and higher efficiency.
Bootstrapping allows the battery voltage to sag to 0.8V
once the system is started. Therefore, the battery volt-
age ranges from (V
forward drop of the Schottky rectifier). If the battery volt-
age exceeds the programmed output voltage, the out-
3.3V/5V or Adjustable-Output,
Step-Up DC-DC Converters
Figure 2a. Standard Application Circuit—Preset Output
Voltage
8
__________________Design Procedure
0.1 F
_______________________________________________________________________________________
R3
R4
C3
*C1 = C2 = 68 F FOR MAX856
C1 = C2 = 22 F FOR MAX858
5
1
3
LBI
SHDN
REF
MAX856/MAX858
OUT
7
GND
Logic Inputs and Outputs
Output Voltage Selection
+ V
C1*
D
OUT
LBO
) to 0.8V (where V
3/5
LX
8
6
2
4
V
IN
1N5817
L1
47 H
OUTPUT
SELECT
D1
R1
.
Do not leave
to GND
D
is the
C2*
–
and
5 to
V
OUT
put will follow the battery voltage. This is acceptable in
many systems; however, the input or output voltage
must not be forced above 7V.
The MAX857/MAX859’s output voltage is set by two
resistors, R1 and R2 (Figure 2b), which form a voltage
divider between the output and FB. Use the following
equation to determine the output voltage:
where V
To simplify resistor selection:
Since the input bias current at FB has a maximum value
of 100nA, large values (10k
for R1 and R2 with no significant accuracy loss. For 1%
error, the current through R1 should be at least 100
times FB’s bias current.
Figure 2b. Standard Application Circuit—Adjustable Output
Voltage
0.1 F
R3
R4
C3
*C1 = C2 = 68 F FOR MAX857
C1 = C2 = 22 F FOR MAX859
REF
V
R1 = R2 ( _______ - 1)
OUT
= 1.25V.
5
1
3
= V
LBI
SHDN
REF
MAX857/MAX859
REF
V
V
OUT
REF
7
GND
( _________ )
R1 + R2
C1*
R2
OUT
LBO
LX
FB
to 300k ) can be used
8
6
2
4
V
IN
1N5817
47 H
L1
D1
R1
R2
C2*
V
OUT