MAX856CSA Datasheet Download

Part No.:
MAX856CSA
Download:
Download Datasheet
Description:
[3.3V/5V or Adjustable-Output, Step-Up DC-DC Converters]
File Size:
128 K
Page:
12 Pages
Logo:
Manufacturer:
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
PCB Prototype
 MAX856CSA Datasheet Page:4MAX856CSA Datasheet Page:5MAX856CSA Datasheet Page:6MAX856CSA Datasheet Page:7MAX856CSA Datasheet Page:9MAX856CSA Datasheet Page:10MAX856CSA Datasheet Page:11MAX856CSA Datasheet Page:12 
3.3V/5V or Adjustable-Output,
Step-Up DC-DC Converters
MAX856–MAX859
Logic Inputs and Outputs
The 3/5 input is internally diode clamped
to GND
and
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
.
Do not leave
control inputs (3/5, LBI, or SHDN) floating.
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:
R1 + R2
V
OUT
= V
REF
( _________ )
R2
where V
REF
= 1.25V.
To simplify resistor selection:
V
OUT
R1 = R2 ( _______ - 1)
V
REF
Since the input bias current at FB has a maximum value
of 100nA, large values (10kΩ to 300kΩ) can be used
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.
__________________Design Procedure
Output Voltage Selection
For the MAX856/MAX858,you can select a 3.3V or 5V
output voltage under logic control, or by tying 3/ 5 to
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
OUT
+ V
D
) to 0.8V (where V
D
is the
forward drop of the Schottky rectifier). If the battery volt-
age exceeds the programmed output voltage, the out-
V
IN
C1*
R3
5
R4
8
C1*
R3
V
OUT
C2*
6
2
4
C3
0.1µF
R1
R4
5
8
V
IN
47µH
L1
D1
1N5817
47µH
L1
D1
1N5817
V
OUT
C2*
6
2
4
R2
R1
LBI
LX
LBI
LX
MAX856/MAX858
OUT
1
3
SHDN
REF
GND
7
3/5
LBO
MAX857/MAX859
OUT
1
3
SHDN
REF
GND
7
FB
LBO
OUTPUT
SELECT
C3
0.1µF
*C1 = C2 = 68µF FOR MAX856
C1 = C2 = 22µF FOR MAX858
*C1 = C2 = 68µF FOR MAX857
C1 = C2 = 22µF FOR MAX859
Figure 2a. Standard Application Circuit—Preset Output
Voltage
8
Figure 2b. Standard Application Circuit—Adjustable Output
Voltage
_______________________________________________________________________________________