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LM3502SQ-16 датащи(PDF) 7 Page - Texas Instruments

номер детали LM3502SQ-16
подробное описание детали  Step-Up Converter for White LED Applications
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LM3502SQ-16 датащи(HTML) 7 Page - Texas Instruments

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LM3502
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SNVS339A – SEPTEMBER 2005 – REVISED AUGUST 2006
comparator will then reset the driver logic, thus turning off the internal NMOS power device, N1, and forward
biasing the external diode. The inductor current then flows through the diode to the white LED load and output
capacitor. The inductor current recharges the output capacitor and supplies the current for the white LED load.
The oscillator then resets the driver logic again repeating the process. The output voltage of the error amplifier
controls the current through the inductor. This voltage will increase for larger loads and decrease for smaller
loads limiting the peak current in the inductor and minimizing EMI radiation. The duty limit comparator is always
operational, it prevents the internal NMOS power switch, N1, from being on for more than one oscillator cycle
and conducting large amounts of current. The light load comparator allows the LM3502 to properly regulate
light/small white LED load currents, where regulation becomes difficult for the LM3502’s primary control loop.
Under light load conditions, the LM3502 will enter into a pulse skipping pulse-frequency-mode (PFM) of operation
where the switching frequency will vary with the load.
The LM3502 has 2 control pins, En1 and En2, used for selecting which segment of a single white LED string
network is active for dual display applications. En1 controls the main display (MAIN) segment of the single string
white LED network between pins VOUT1 and VOUT2. En2 controls the sub display (SUB) segment of the single
string white LED network between the VOUT2 and Fb. For a quick review of the LM3502 control pin operational
characteristics, see Figure 5.
When the Cntrl pin is
≥ 1.4V, the LM3502 will enter in low IQ state if both En1 and En2 ≤ 0.3V. At this time, both
the P1 and N2 FETs will turn off. The output voltage will be a diode drop below the supply voltage and the soft-
start will be reset limiting the peak inductor current at the next start-up.
The LM3502 is designed to control the LED current with a PWM signal without the use of an external RC filter.
Utilizing special circuitry, the LM3502 can operate over a large range of PWM frequencies without restarting the
soft-start allowing for fast recovery at high PWM frequencies. Figure 6 reprsents a PWM signal driving the Cntrl
pin where tL is defined as the low time of the signal. The following is true:
If tL < 12ms (typical): The device will stop switching during this time and the soft-start will not be reset allowing
LED current PWM control.
If tL > 12ms (typical): The device will shutdown and the soft-start will reset to prevent high peak currents at the
next startup. Both the N2 and P1 switches will turn off.
The LM3502 has dedicated protection circuitry active during normal operation to protect the integrated circuit (IC)
and external components. The thermal shutdown circuitry turns off the internal NMOS power device, N1, when
the internal semiconductor junction temperature reaches excessive levels. The LM3502 has a under-voltage
protection (UVP) comparator that disables the internal NMOS power device when battery voltages are too low,
thus preventing an on state where the internal NMOS power device conducts large amounts of current. The over-
voltage protection (OVP) comparator prevents the output voltage from increasing beyond the protection limit
when the white LED string network is removed or if there is a white LED failure. OVP allows for the use of low
profile ceramic capacitors at the output. The current though the internal NMOS power device, N1, is monitored to
prevent peak inductor currents from damaging the IC. If during a cycle (cycle=1/switching frequency) the peak
inductor current exceeds the current limit for the LM3502, the internal NMOS power device will be turned off for
the remaining duration of that cycle.
Copyright © 2005–2006, Texas Instruments Incorporated
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