AMC7150_1 Datasheet, PDF (5/7 Pages) ADDtek Corp – 1.5A POWER LED DRIVER

AMC7150
Inductor
The Inductor L stores energy during switch turn-on period and discharge driving current to LEDs via flywheel
diode while switch turn-off. In order to reduce the current ripple on LEDs, the L value should high enough to keep the
system working at continuous-conduction mode that inductor current won’t fall to zero.
Since in steady-state operation the waveform must repeat from one time period to the next, the integral of the
inductor voltage v_L over one time period must be zero:
∫ ∫ ∫ ^Ts
0
v _L dt
=
t_ON
0
v_L
dt
+
T_s
t_ON
v _L dt
=
0
Where T_s = t_ON + t_OFF
Therefore
t_{ON =}
V_LED + V_F
t_OFF V_CC − V_Rsense − V_SAT − V_LED
Where, V_LED is the total forward voltage (at expecting current) of the LED string, V_F is the forward voltage of the
flywheel diode D_F, V_Rsense is the peak value of the voltage drop across Rsense which is 300mV, and V_SAT is the saturation
voltage of the switch which has a typical value of 1V.
Since the operation frequency f is determined by choosing appropriate value for timing capacitor C_T, the switch
turn-on time can also be known by
t_ON
=
D ⋅T_s
=
D
f
Where D(Dutycycle) = ^tON
t_ON + t_OFF
With knowledge of the peak switch current and switch on time, the value of inductance can be calculated.
L
= ^VCC
− V_Rsense − V_SAT
I _PK
− V_LED
⋅ t_ON
Copyright © 2006 ADDtek Corp.
5
DD034_A -- JUNE 2006