Datasheet MAX1573 (Analog Devices) - 6

White LED 1x/1.5x Charge Pump in UCSP and Thin QFN True Shutdown™ Mode Applications Information
When EN1 and EN2 are grounded, the MAX1573 is in shutdown, and the charge pump examines whether the
Dimming Using EN1 and EN2
input voltage is greater than or less than the output volt- Use EN1 and EN2 inputs as a digital 2-bit number to age and shorts the transfer capacitor nodes to either IN control on/off, 1/10, 3/10, and full current (see Table 1). or OUT as necessary. The output is high impedance in RSET programs the full current level (see the Setting the either case. Output Current section).
Thermal Shutdown Dimming Using PWM into EN1 MAX1573
The MAX1573 includes a thermal-limit circuit that shuts Use EN2 for shutdown and drive EN1 with a PWM sig- down the IC at about +160°C. Turn-on occurs after the nal. Current can be varied from 1/10 to full. The wave- IC cools by approximately 20°C. forms in the Typical Operating Characteristics show the response time of dimming. EN2 keeps the part on, elimi-
Setting the Output Current
nating any soft-start delay that would impede PWM con- SET controls the LED bias current. Current flowing into trol, allowing a PWM frequency up to 50kHz (Figure 1). LED1, LED2, LED3, and LED4 is a multiple of the current flowing out of SET. Set the output current as follows:
Dimming Using a Filtered PWM Signal
Use a high-frequency PWM signal to drive an R-C-R fil- . 0 V 6 ter on the SET pin (Figure 2). A 0% PWM duty cycle ⎞ I = ⎛ K x LED _ corresponds to 18.7mA/LED, while a 100% PWM duty ⎝⎜ RSET⎠⎟ cycle corresponds to 0mA/LED. At PWM frequencies above 5kHz, C3 may be reduced. where K = 22, 66, or 219 (depending upon EN1 and EN2, see Table 1), and RSET is the resistor connected
Input Ripple
between SET and GND (see the Typical Operating For LED drivers, input ripple is more important than out- Circuit). put ripple. Input ripple depends on the source supply’s impedance. Adding a lowpass filter to the input further
Table 1. EN1/EN2 States
reduces input ripple. Figure 3 shows a C-R-C filter used to reduce input ripple to less than 2mVP-P when driving
EN1/EN2 STATES BRIGHTNESS LED CURRENT
a 75mA load. Alternately, increasing CIN to 2.2µF or EN1 = low, EN2 = low Shutdown ILED = 0 4.7µF yields input ripple of 17mVP-P or 9mVP-P, respec- EN1 = low, EN2 = high 1/10 Brightness ILED = 22 x ISET tively, with only a small increase in footprint. The 1x EN1 = high, EN2 = low 3/10 Brightness ILED = 66 x ISET mode always has very low input ripple. EN1 = high, EN2 = high Full Brightness ILED = 219 x ISET True Shutdown is a trademark of Maxim Integrated Products, Inc. 1μF 1μF 1μF 1μF C1P C1N C2P C2N C1P C1N C2P C2N IN OUT IN OUT 0 TO 18.7mA/LED VIN VIN 1μF 1μF 1μF 1μF MAX1573 MAX1573 EN2 LED1 EN1 LED1 EN1 LED2 EN2 LED2 PWM INPUT R1 R2 PWM 14.3k SET LED3 Ω 14.3kΩ SET LED3 INPUT 0 TO LED4 LED4 2.5V C3 R R SET SET 1μF 9.09kΩ GND GND Figure 1. Dimming Using PWM Signal into EN1 Figure 2. Dimming Using Filtered PWM Signal
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