TB62731FUG データシートの表示（PDF） - Marktech Optoelectronics

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TB62731FUG

Step-up DC-DC Converter for White LED Driver

Marktech Optoelectronics 

TB62731FUG

Pin A voltage

Maximum duty width for

inductor on: 83.3% of fOSC

Pulse output time

width fOSC = 350 kHz

Pulse stop time width:

7.5 µs (min)~15 µs (max)

Repetition of waveforms at left

Pin A current (external

inductance current)

I A (peak)

= 240 mA (typ.)

Pin K voltage (current

charged on capacitor)

Figure 3 Burst Control Waveforms

Burst Control

Burst control is control of the number of current pulses, shown in the graph on the previous page.

Control is repeated in desired cycles. The current pulse in the graph is the charged current on capacitor 2 (C2)

for output.

The current pulse is supplied to the LED as current discharged from the output-side capacitor. The current

pulse flows to GND via R_sens.

The waveform of the voltage charged on the output-side capacitor is fed back to the IC from pin K via C2.

The internal circuit which uses pin K for input controls the number of current pulses so that the mean voltage

value of the obtained voltage waveform is 36 mV. As a result, the output current is controlled as the constant

current (= mean current).

Connecting R_sens = 1.8 Ω obtains the mean current (36 mV ÷ 1.8 Ω = 20 mA).

Current is controlled by PFM (pulse frequency modulation) because the time when the output pulse is

generated varies (increases/decreases).

A prerequisite is that the input power from VIN is larger than the output power to the LED load. The

constant current is maintained by fixing a pulse stop time of 7.5~15 µs and increasing/decreasing the number of

current pulses. The number of current pulses is fewer when the input power exceeds the output power, larger

when the input power is less than the output power.

The burst frequency (pulse generation frequency) at controlled constant current is calculated as follows:

fburst [Hz] = (number of current pulses x (1/275~1/350 kHz) + pulse stop time (7.5~15 µs) . . . formula 1

The IC is designed to supply a load power of 320 mW (min).

Generally, a step-up inductance of 47 µH is used for optimum design for the load power of 320 mW. When the

load power is small, the inductance must be small.

Make sure the following condition for LED load between pins A and K is satisfied:

VIN (VCC) < LED Vf total

Note that, regardless of control by the IC, LEDs are always on.

6

2004-09-14

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