MAX1595_09 Datasheet, PDF (5/7 Pages) Maxim Integrated Products – Regulated 3.3V/5.0V Step-Up/Step-Down Charge Pump

Regulated 3.3V/5.0V Step-Up/Step-Down
Charge Pump
Applications Information
Using white LEDs to backlight LCDs is an increasingly
popular approach for portable information devices
(Figure 2). Because the forward voltage of white LEDs
exceeds the available battery voltage, the use of a
charge pump such as the MAX1595 provides high effi-
ciency, small size, and constant light output with chang-
ing battery voltages. If the output is used only to light
LEDs, the output capacitor can be greatly reduced. The
frequency modulation of the LED intensity is not dis-
cernible to the human eye, and the smaller capacitor
saves both size and cost.
Adding two Schottky diodes and two capacitors imple-
ments a tripler and allows the MAX1595_ _ _50 to regu-
late a current of 75mA with a supply voltage as low as
2.3V (Figure 3).
Capacitor Selection
The MAX1595 requires only three external capacitors
(Figure 4). Their values are closely linked to the output
current capacity, oscillator frequency, output noise con-
tent, and mode of operation.
Generally, the transfer capacitor (C_X) will be the smallest,
and the input capacitor (C_IN) is twice as large as C_X.
Higher switching frequencies allow the use of the smaller
C_X and C_IN. The output capacitor (C_OUT) can be any-
where from 5-times to 50-times larger than C_X. Table 1
shows recommended capacitor values.
In addition, the following equation approximates output
ripple:
V_RIPPLE ≅ I_OUT / (2 x f_OSC x C_OUT)
Table 2 lists the manufacturers of recommended capaci-
tors. Ceramic capacitors will provide the lowest ripple
due to their typically lower ESR.
Power Dissipation
The power dissipated in the MAX1595 depends on out-
put current and is accurately described by:
P_DISS = I_OUT (2V_IN - V_OUT)
P_DISS must be less than that allowed by the package
rating.
Layout Considerations
All capacitors should be soldered in close proximity to
the IC. Connect ground and power ground through a
short, low-impedance trace. The input supply trace
should be as short as possible. Otherwise, an additional
input supply filter capacitor (tantalum or electrolytic) may
be required.
V_IN
C_IN = 1μF
C_X = 0.1μF
CXP
CXN
MAX1595_ _ _50
IN
OUT
SHDN
AOUT
PGND GND
C_OUT =
0.47μF
100Ω
100Ω
100Ω
Figure 2. White LED Bias Supply
INPUT
2.3V
1μF
IN
SHDN
AOUT
OUT
MAX1595_ _ _50
CXP
PGND GND ^CXN
OUTPUT
REGULATED 5V
1μF
0.22μF
1μF ^75mA
0.22μF
Figure 3. Regulated Voltage Tripler
ON
OFF
IN
C_IN
1μF
2
SHDN
3
IN
MAX1595
PGND GND
5
4
7
CXP
CXN ⁶
8
OUT
1
AOUT
C_X
0.22μF
OUT
C_OUT
1μF
Figure 4. Standard Operating Circuit
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