Datasheet MAX1595 (Maxim) - 5

Regulated 3.3V/5.0V Step-Up/Step-Down Charge Pump MAX1595 Applications Information
Using white LEDs to backlight LCDs is an increasingly CX = 0.1µF popular approach for portable information devices (Figure 2). Because the forward voltage of white LEDs CXP CXN exceeds the available battery voltage, the use of a charge pump such as the MAX1595 provides high effi- MAX1595_ _ _50 ciency, small size, and constant light output with chang- VIN IN OUT ing battery voltages. If the output is used only to light AOUT LEDs, the output capacitor can be greatly reduced. The SHDN COUT = frequency modulation of the LED intensity is not dis- CIN = 1µF 0.47µF 100Ω 100Ω 100Ω cernible to the human eye, and the smaller capacitor PGND GND 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). Figure 2. White LED Bias Supply
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 (CX) will be the smallest, and the input capacitor (CIN) is twice as large as CX. INPUT IN AOUT Higher switching frequencies allow the use of the smaller 2.3V OUTPUT SHDN OUT REGULATED 5V CX and CIN. The output capacitor (COUT) can be any- 1µF 0.22µF 1µF 75mA 1µF where from 5-times to 50-times larger than CX. Table 1 MAX1595_ _ _50 shows recommended capacitor values. CXP 0.22µF In addition, the following equation approximates output CXN ripple: PGND GND VRIPPLE ≅ IOUT / (2 x fOSC x COUT) Table 2 lists the manufacturers of recommended capaci- Figure 3. Regulated Voltage Tripler 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 7 DISS = IOUT (2VIN - VOUT) CXP ON 2 CX SHDN P OFF 6 DISS must be less than that allowed by the package 0.22µF CXN 3 rating. IN IN MAX1595 8 OUT OUT
Layout Considerations
1 CIN AOUT All capacitors should be soldered in close proximity to PGND GND 1µF COUT 5 4 the IC. Connect ground and power ground through a 1µF 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. Figure 4. Standard Operating Circuit
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