Datasheet MAX1573 (Analog Devices) - 5

White LED 1x/1.5x Charge Pump in UCSP and Thin QFN MAX1573 Pin Description (continued) PIN NAME FUNCTION TQFN UCSP
13 D1 C1N Transfer Capacitor 1 Negative Connection. See the Component Selection section for capacitor value. 14 C1 IN Supply Voltage Input. IN supplies power for the IC. Bypass IN to GND with a 1µF ceramic capacitor. 15 B1 C2N Transfer Capacitor 2 Negative Connection. See the Component Selection section for capacitor value. 16 A1 C1P Transfer Capacitor 1 Positive Connection. See the Component Selection section for capacitor value. The MAX1573 utilizes a proprietary 1x/1.5x fractional
Block Diagram
charge-pump topology to drive up to four white LEDs with regulated constant current for uniform intensity. The MAX1573 operates with a 1MHz fixed frequency. C1P C1N C2P C2N An external resistor (RSET) programs the full-scale LED IN OUT 1x/1.5x REGULATED CHARGE PUMP current, while two digital inputs control on/off and pro- vide brightness control.
Output Regulation
The MAX1573 operates in 1x mode until just above dropout. Then the MAX1573 switches to 1.5x charge- EN1 0.15V 1.25V pump mode to regulate the lowest of LED1–LED4 to BRIGHTNESS 150mV and maintain constant LED brightness even at CONTROL EN2 0.6V very low battery voltages. Using this topology, there is MINIMUM no LED brightness change during the 1x/1.5x switchover, SELECT which guarantees no flicker on the display. The P P switchover scheme has low hysteresis, minimizing operation in the less-efficient 1.5x mode. The 1x mode LED1 LDO CURRENT produces almost no ripple, while the 1.5x mode regu- REGULATOR lates the output voltage by controlling the rate at which N the transfer capacitors are charged. In this way, the switching frequency remains constant for reduced input LED2 LDO CURRENT REGULATOR ripple and stable noise spectrum. SET
Soft-Start
LED3 The MAX1573 includes soft-start circuitry to limit inrush LDO CURRENT REGULATOR current at turn-on. When starting up with an output voltage that is not near the input voltage, the output MAX1573 capacitor is charged directly from the input with a DAC- LED4 LDO CURRENT ramped current source (with no charge-pump action) REGULATOR until the output voltage is near the input voltage. Once this occurs, the charge pump determines if 1x or 1.5x GND mode is required. In the case of 1x mode, the soft-start is terminated and normal operation begins. In the case of 1.5x mode, soft-start operates until the lowest of LED1–LED4 reaches regulation. If an overload condi-
Detailed Description
tion occurs, soft-start repeats every 2.1ms. If the output The MAX1573 is a complete charge-pump buck-boost is shorted to ground, the output current is limited by converter requiring only four small ceramic capacitors. the MAX1573 fractional-switching technique.
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