tb62731fu TOSHIBA Semiconductor CORPORATION, tb62731fu Datasheet - Page 6

• Manufacturer Part Number: tb62731fu
• Description: Step-up Dc-dc Converter For White Led Driver
• Manufacturer: TOSHIBA Semiconductor CORPORATION

Burst Control
Control is repeated in desired cycles. The current pulse in the graph is the charged current on capacitor 2 (C2)
for output.
pulse flows to GND via R_sens.
value of the obtained voltage waveform is 36 mV. As a result, the output current is controlled as the constant
current (= mean current).
generated varies (increases/decreases).
constant current is maintained by fixing a pulse stop time of 7.5~15 µs and increasing/decreasing the number of
current pulses. When the input power is more than the output power, the number of current pulses is fewer;
when less, the number of pulses is larger.
load power is small, the inductance must be small.
Burst control is control of the number of current pulses, shown in the graph on the previous page.
The current pulse is supplied to the LED as current discharged from the output-side capacitor. The current
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
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
A prerequisite is that the input power from V
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
Make sure the following condition for LED load between pins A and K is satisfied.
Note that, regardless of control by the IC, LEDs are always on.
Pin A voltage
Pin A current (external
inductance current)
Pin K voltage (current
charged on capacitor)
V
IN
(V
CC
) < LED Vf total
Pulse output time
width f
Figure 3 Burst Control Waveforms
OSC
Maximum duty width for
inductor on: 83.3% of f
= 350 kHz
I A (peak)
= 240 mA (typ.)
IN
is larger than the output power to the LED load. The
OSC
6
Pulse stop time width:
7.5 µs (min)~15 µs (max)
Repetition of waveforms at left
TB62731FU
2006-06-14