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FAN5617 Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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FAN5617 Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 10 page © 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5617 Rev. 1.0.1 Circuit Description The FAN5617 switch capacitor DC/DC converter automatically configures its internal switches to achieve high efficiency, providing tightly-regulated output currents for the LEDs (see Figure 10 and Figure 11). An internal voltage detector determines which LED requires the highest voltage to sustain the pre-set current levels and adjusts the charge pump regulator accordingly. Each LED has its own linear current regulator. In addition, a voltage regulator controls the output voltage when the battery voltage is within a range where linear regulation can provide maximum efficiency. If the battery voltage is too low to sustain the LED current in linear mode, a fractional 3:2 charge pump is enabled. When the battery voltage drops and the mode can no longer sustain proper operation, the pump is automatically reconfigured to operate in 2:1 mode. As the battery discharges and the voltage decays, the FAN5617 switches between modes to maintain a constant current through the LEDs throughout the battery life. These transitions have hysteresis to prevent toggling. The internal supply voltage of the device is automatically selected from the VIN or VOUT pins, whichever has a higher voltage. The FAN5617 enters shutdown mode to reduce overall current consumption when the chip is disabled by the EN pin and control register. Start-up When the FAN5617 is enabled, either by EN going high or on command from the TinyWire™ interface, the soft- start circuit linearly ramps VOUT to limit the switcher’s inrush current (see Figure 12). When power is first applied, the default IC brightness control register (NBR) is set to maximum brightness (31). If EN is low at initial power-up, the IC remains in a low-power state until either EN goes HIGH or a non-0 brightness command is received. If EN goes from HIGH to LOW, the IC is reset to the low-power state until either a new brightness command is received or EN is driven HIGH. V IN V OUT PWRUP 250 ∝s Figure 12. Soft-Start An internal PWRUP signal turns on all of the IC’s logic control circuitry and starts the charge pump. PWRUP is high under any of the following conditions: 1. EN goes from low to high 2. A non-zero brightness command is received PWRUP goes low under any of the following conditions: 1. EN is low when power is first applied and no subsequent non- zero brightness commands are received 2. EN goes from HIGH to LOW 3. A zero brightness command is received TinyWire™ Single-Wire Brightness Control This IC features a digitally controlled static brightness adjustment to minimize both EMI and applications processor overhead. The brightness command consists of a 5-bit binary word (NBR). LED current is a multiple of the current through RSET: ( ) SET BR LED R 32 1 N 156 I • + • = for NBR > 0 (1) NBR BIT4 BIT3 BIT2 BIT1 BIT0 ILED/ISET 0 0 0 0 0 0 0 1 0 0 0 0 1 260 × 2/32 2 0 0 0 1 0 260 × 3/32 3 0 0 0 1 1 260 × 4/32 4 0 0 1 0 0 260 × 5/32 5 0 0 1 0 1 260 × 6/32 6 0 0 1 1 0 260 × 7/32 7 0 0 1 1 1 260 × 8/32 8 0 1 0 0 0 260 × 9/32 9 0 1 0 0 1 260 × 10/32 10 0 1 0 1 0 260 × 11/32 11 0 1 0 1 1 260 × 12/32 12 0 1 1 0 0 260 × 13/32 13 0 1 1 0 1 260 × 14/32 14 0 1 1 1 0 260 × 15/32 15 0 1 1 1 1 260 × 16/32 16 1 0 0 0 0 260 × 17/32 17 1 0 0 0 1 260 × 18/32 18 1 0 0 1 0 260 × 19/32 19 1 0 0 1 1 260 × 20/32 20 1 0 1 0 0 260 × 21/32 21 1 0 1 0 1 260 × 22/32 22 1 0 1 1 0 260 × 23/32 23 1 0 1 1 1 260 × 24/32 24 1 1 0 0 0 260 × 25/32 25 1 1 0 0 1 260 × 26/32 26 1 1 0 1 0 260 × 27/32 27 1 1 0 1 1 260 × 28/32 28 1 1 1 0 0 260 × 29/32 29 1 1 1 0 1 260 × 30/32 30 1 1 1 1 0 260 × 31/32 31 1 1 1 1 1 260 × 32/32 Table 1. NBR Data Dimming Values |
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