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LTC3452EUF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3452EUF Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LTC3452 3452f LED Current Setting Amps The maximum forward current per LED for all LEDs in a given bank is programmed by a single external resistor to ground at the corresponding ISETL,H pin according to the following formulas: I R I R MAXL ISETL MAXH ISETH = ⎛ ⎝⎜ ⎞ ⎠⎟ = ⎛ ⎝⎜ ⎞ ⎠⎟ 256 08 768 08 . , . For operation at currents below IMAXL in the low power bank, refer to the Exponential Brightness Control section and also to external circuit options given in the Applica- tions Section. For operation at currents below IMAXH in the high power bank, refer only to the external circuit options given in the Applications Section. Shutdown Circuit The shutdown circuit monitors the voltages at the ENL,H pins. Logic high on either/both inputs enables the part and logic low on both puts the part in shutdown. Since the ENL pin doubles as a PWM input for LED brightness control, an output filter in the shutdown circuit is employed to prevent the part from toggling in and out of shutdown for normal PWMing of the ENL input when ENH is low. If ENH is low, the LTC3452 is enabled immediately after a rising edge at the ENL pin, but waits 200µs (typical) after a falling edge to enter shutdown. Consequently, a minimum PWM fre- quency is required for smooth brightness control at cur- rents below IMAXL. The recommended PWM frequency is 10kHz to 50kHz. Exponential Brightness Control (Low Power LED Bank Only) The LTC3452 implements an exponential brightness con- trol function for the low power LED bank only in which the LEDLx current is a function of the PWM duty cycle at the ENL pin. The LED current will be equal to a fraction of IMAXL as given in Table 1. As the duty cycle (that the PWM waveform is logic high) increases linearly, the LED cur- rent will increase exponentially from 1/128th IMAXL to 128/128ths IMAXL in seven binary steps. The function implemented results in “smoother” brightness and dim- ming control as perceived by the human eye, which is logarithmic in nature. Table 1. Low Power Bank Brightness Control ENL DUTY CYCLE (% LOGIC HIGH) LEDLx CURRENT 0% (Logic Low) 0 (Shutdown) 0% < Duty Cycle < 12.5% 1/128 • IMAXL 12.5% < Duty Cycle < 25% 1/64 • IMAXL 25% < Duty Cycle < 37.5% 1/32 • IMAXL 37.5% < Duty Cycle < 50% 1/16 • IMAXL 50% < Duty Cycle < 62.5% 1/8 • IMAXL 62.5% < Duty Cycle < 75% 1/4 • IMAXL 75% < Duty Cycle < 87.5% 1/2 • IMAXL 87.5% < Duty Cycle ≤ 100% IMAXL LED Current Sources Each LED pin is driven by a current source specifically designed for low dropout. The LTC3452 employs a propri- etary architecture that determines which of the seven LEDs requires the largest forward voltage drop at its programmed current, and then generates a feedback voltage based on this one for closing the buck-boost regulation loop. This results in the lowest output voltage required for regulating all of the LEDs and thus the highest LED power efficiency. The voltage present at the LED pin of the “controlling LED” will be typically 130mV at 20mA (low power bank) or 250mA at 100mA (high power bank) of current. LED Detect Circuit If fewer than five LED outputs in the low power bank and/ or fewer than two LED outputs in the high power bank are required, unused outputs should be connected to VOUT. Each LED pin has an internal LED detect circuit that disables the output current source to save power if an output is not needed. A small current is employed to detect the presence of an LED at startup. This current is typically 10 µA for the low power bank and 30µA for the high power bank. OPERATIO |
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