ZLED7730 Datasheet, PDF (17/22 Pages) List of Unclassifed Manufacturers – High Current 40V LED Driver with Switch Dimming

ZLED7x30 Datasheet
The inductance value has an equivalent effect on t_ON and t_OFF and therefore affects the switching frequency. For the
same reason, the inductance has no influence on the duty cycle, for which the relationship of the summed LED
forward voltages n * V_F to the input voltage V_IN is a reasonable approximation. Because the input voltage is a factor in
the ON time, variations in the input voltage affect the switching frequency and duty cycle.
To achieve optimum performance, duty cycles close to 0.5 at the nominal average supply voltage are preferable for
improving the temperature stability of the output current.
Equations (5), (6), (7), and (8) provide an example of calculating t_ON, t_OFF, operating frequency f_LX, and duty cycle D_LX
when using a 220μH inductor for L1 and V_IN=12V, Rs = 0.30Ω, r_L=0.26Ω, V_LED=3.4V, I_AVG =333mA, V_D=0.36V, and
R_LX=0.27Ω.
Example:
t_ON
=
12V
− 3.4V
220µH ∗ 0.3 ∗ 0.333A
− 0.333A ∗ (0.3Ω + 0.26Ω +
0.27Ω)
= 2.64µs
(5)
t_OFF
=
3.4V
+
220µH ∗ 0.3 ∗ 0.333A
0.36V + 0.333A ∗ (0.30Ω
+
0.26Ω)
=
5.56µs
(6)
f _LX
=
T_ON
1
+ T_OFF
=
1
2.64µs + 5.56 µs
= 121.8kHz
(7)
D_LX
= ^VLED
V_IN
= ^3.4V
12V
≈ ^TON
T_ON + T_OFF
= ^2.64µs
2.64µs + 5.56 µs
≈ 0.3
(8)
For the L1 inductor, use a coil with a continuous current rating higher than the required mean output current and a
saturation current that exceeds the peak output current by 30% to 50% for robustness against transient conditions;
e.g., during start-up.
3.3.3 Bypass Capacitor C1
The bypass capacitor C1 has two functions: maintaining operating voltage and bypassing the current ripple of the
switching converter. In general, low ESR capacitors must be used.
If the circuit is supplied by rectified line voltage, C1 must provide enough charge to maintain the ZLED7x30’s
minimum operating voltage as well as the forward voltage of the LED string to keep the application working even if
the rectified supply voltage periodically drops below these values. A rough estimate for the minimum capacity needed
can be calculated with equation (9).
© 2016 Integrated Device Technology, Inc.
17
April 20, 2016