MAX16812 Datasheet, PDF (15/21 Pages) Maxim Integrated Products – Integrated High-Voltage LED Driver with Analog and PWM Dimming Control

Integrated High-Voltage LED Driver
with Analog and PWM Dimming Control
TO L_REG PIN
OF MAX16812
GND
TOVER
GND
MAX6501
V_CC
V_IN
100kΩ
5.1V
ZENER
TO EN PIN OF
MAX16812
4.7µF
Figure 12. Dimming MOSFET Protection
Inductor Selection
The minimum required inductance is a function of the
operating frequency, the input-to-output voltage differ-
ential and the peak-to-peak inductor current (∆I_L).
Higher ∆I_L allows for a lower inductor value while a
lower ∆I_L requires a higher inductor value. A lower
inductor value minimizes size and cost, improves large-
signal transient response, but reduces efficiency due to
higher peak currents and higher peak-to-peak output
ripple voltage for the same output capacitor. On the
other hand, higher inductance increases efficiency by
reducing the ripple current, ∆I_L. However, resistive
losses due to the extra turns can exceed the benefit
gained from lower ripple current levels, especially when
the inductance is increased without allowing for larger
inductor dimensions. A good compromise is to choose
∆I_L equal to 30% of the full load current. The inductor
saturating current specification is also important to
avoid runaway current during output overload and con-
tinuous short-circuit conditions.
Buck Configuration: In a buck configuration (Figure
13), the average inductor current does not vary with the
input. The worst-case peak current occurs at the high-
est input voltage. In this case, the inductance, L, for
continuous conduction mode is given by:
( ) L = VOUT x VINMAX − VOUT
V_INMAX x f_SW x ∆I_L
where V_INMAX is the maximum input voltage, f_SW is the
switching frequency, and V_OUT is the output voltage.
Boost Configuration: In the boost converter, the aver-
age inductor current varies with the input voltage and
the maximum average current occurs at the lowest
input voltage. For the boost converter, the average
inductor current is equal to the input current. In this
case, the inductance, L, is calculated as:
( ) L = VINMIN x VOUT − VINMIN
V_OUT x f_SW x ∆I_L
where V_INMIN is the minimum input voltage, V_OUT is the
output voltage, and f_SW is the switching frequency. See
Figure 14.
Buck-Boost Configuration: In a buck-boost converter
(see the Typical Application Circuit), the average
inductor current is equal to the sum of the input current
and the LED current. In this case, the inductance, L, is:
( ) ^{L =}
V_OUT x V_INMIN
V_OUT + V_INMIN x f_SW x ∆I_L
where V_INMIN is the minimum input voltage, V_OUT is the
output voltage, and f_SW is the switching frequency.
______________________________________________________________________________________ 15