Applications information – Rainbow Electronics MAX16822B User Manual
Page 7
Undervoltage Lockout (UVLO)
The MAX16822A/MAX16822B include UVLO with
500mV hysteresis. The internal MOSFET turns off when
V
IN
falls below 5.5V to 6.0V.
DIM Input
LED dimming is achieved by applying a PWM signal at
DIM. A logic level below 0.6V at DIM forces the
MAX16822A/MAX16822B’s output low, thus turning off
the LED current. To turn the LED current on, the logic
level at DIM must be greater than 2.8V.
Thermal Shutdown
The MAX16822A/MAX16822B thermal-shutdown fea-
ture turns off the LX driver when the junction tempera-
ture exceeds +165°C. The LX driver turns back on
when the junction temperature drops 10°C below the
shutdown-temperature threshold.
Analog Dimming
The MAX16822A/MAX16822B offer an analog dimming
feature that reduces the output current when the volt-
age at TEMP_I is below the internal 2V threshold volt-
age. The MAX16822A/MAX16822B achieve analog
dimming by either an external DC voltage source con-
nected between TEMP_I and ground or by a voltage on
a resistor connected across TEMP_I and ground
induced by an internal current source of 25µA. When
the voltage at TEMP_I is below the internal 2V threshold
limit, the MAX16822A/MAX16822B reduce the LED cur-
rent. Use the following formula to set the analog dim-
ming current.
where V
TFB_ON
= 2V and FB
SLOPE
= 0.75 are obtained
from the
Electrical Characteristics
table and V
AD
is the
voltage at TEMP_I.
Thermal Foldback
The MAX16822A/MAX16822B also include a thermal-
foldback feature that reduces the output current when
the temperature of the LED string exceeds a specified
temperature point. These devices enter thermal-fold-
back mode when the voltage drop on the NTC thermis-
tor, thermally attached to the LEDs and electrically
connected between TEMP_I and ground, drops below
the internal 2V threshold limit.
Applications Information
Selecting R
SENSE
to Set LED Current
The LED current is programmed with a current-sense
resistor connected between IN and CS. Use the equa-
tion below to calculate the value of this resistor:
where V
SNSHI
is the sense voltage threshold high and
V
SNSLO
is sense voltage threshold low (see the
Electrical Characteristics
table for values).
Current-Regulator Operation
The MAX16822A/MAX16822B regulate the LED current
using a comparator with hysteresis (see Figure 2). As
the current through the inductor ramps up and the volt-
age across the sense resistor reaches the upper
threshold, the internal MOSFET turns off. The internal
MOSFET turns on again when the inductor current
ramps down through the freewheeling diode until the
voltage across the sense resistor equals the lower
threshold. Use the following equation to determine the
operating frequency:
where n is the number of LEDs, V
LED
is the forward
voltage drop of 1 LED, and ΔV = (V
SNSHI
- V
SNSLO
).
Inductor Selection
The MAX16822A/MAX16822B operate up to a switching
frequency of 2MHz. For space-sensitive applications,
the high switching frequency allows the size of the
inductor to be reduced. Use the following formula to
calculate an approximate inductor value and use the
closest standard value:
For component selection, use the MAX16822A/B Design
Tool available at:
www.maximic.com/MAX16822-
software
.
L approx
V
nV
nV
R
V
V
f
IN
LED
LED
SENSE
IN
SW
(
.)
(
)
=
Ч
Ч
Ч
Ч
−
Δ
f
V
nV
nV
R
V
V L
SW
IN
LED
LED
SENSE
IN
=
Ч
Ч
Ч
Ч
−
(
)
Δ
R
V
V
V
I
A
SENSE
SNSHI
SNSLO
LED
( )
(
)( )
( )
Ω =
+
1
2
I
A
I
A
FB
V
V
V
V
TF
LED
SLOPE
TFB ON
AD
( )
( )
( )
_
=
Ч
⎛
⎝⎜
⎞
⎠⎟
Ч
(
)
⎡
⎣
⎢
⎤
⎦
⎥
−
−
1
1
MAX16822A/MAX16822B
2MHz, High-Brightness LED Drivers with
Integrated MOSFET and High-Side Current Sense
_______________________________________________________________________________________
7