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RT8532GQW Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT8532GQW Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 14 page  10 RT8532 www.richtek.com DS8532-04 January 2014 © Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. LED ISET 240 ImA Rk where RISET is the resistor between the ISET pin and GND. This setting is the reference for the LED current at LED1 to LED6 and represents the sensed LED current for each string. The DC/DC converter regulates the LED current according to the setting. PWM Mode and Mixed Mode Brightness Dimming The RT8532 allows two ways of controlling the LED brightness. PWM Mode Dimming : When the MIX pin is connected to GND, the dimming mode operates in PWM Mode. During the PWM dimming, the current source turn-on/off is synchronized with the PWM signal. The LED current frequency is equivalent to PWM input frequency. Mixed Mode Dimming : If the MIX pin is floating or tied to VDC, the dimming mode operates in Mixed Mode. In this mode the PWM and ILED dimming cycle will delay by 2 periods. First cycle delay is required for the period, while the second cycle delay is for the duty rate calculation. (a) When 25% ≤ PWM duty ≤ 100%, the current source outputs are DC dimming, and the PWM duty cycle modulates the amplitude of the currents. (b) When PWM Duty < 25%, the DC dimming will translate to DC-PWM dimming to control the LED current. In this state, the LED current is fixed at 0.25 x ISET, and the dimming duty is 4 x PWM duties. The minimum D/A Converter is 512 steps resolution for ILED regulation. where VOUT is the maximum output voltage, VIN is the minimum input voltage, fSW is the switching frequency, and IOUT is the sum of current from all LED strings. LED Soft-Start Function The soft-start time of the LED boost converter, defined as the period from EN to set IOUT, is several tens of milliseconds according to the difference of PWM or Mixed mode. The LED starts up after VIN, PWM and EN signals are all ready. The soft-start inrush peak current must be less than 2.5A. LED Driver Compensation The control loop can be compensated by adjusting the external components connected to the COMP pin. The COMP pin is the output of the internal error amplifier. The compensation capacitors, CC and CCOMP, will adjust the integrator zero and pole respectively to maintain stability. Moreover, the resistor, RCOMP, will adjust the mid-band gain for fast transient response. Diode Selection Schottky diodes D1 are recommended for most applications because of their fast recovery time and low forward voltage. Power dissipation, reverse voltage rating, and pulsating peak current are important parameters for consideration when making a Schottky diode selection. Make sure that the diode's peak current rating exceeds IPEAK and reverse voltage rating exceeds the maximum output voltage. 2 IN OUT IN 2 OUT OUT SW VD V V L1 0.4 V I f where η is the efficiency of the boost converter, IIN(MAX) is the maximum input current and IRIPPLE is the inductor ripple current. The input peak current can be obtained by adding the maximum input current with half of the inductor ripple current as shown in the following equation : IPEAK = 1.2 x IIN(MAX) Note that the saturated current of inductor must be greater than IPEAK. The inductance can eventually be determined according to the following equation : Setting and Regulation of LED Current The LED current can be calculated by the following equation : |
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