AAT1407 Advanced Analogic Technologies, Inc., AAT1407 Datasheet - Page 11

• Manufacturer Part Number: AAT1407
• Description: 6 Channel Led Backlight Driver With Integrated Boost And High Frequency Direct Pwm Dimming
• Manufacturer: Advanced Analogic Technologies, Inc.

Application Information
OVP Protection with Open LED Failure
The AAT1407's OVP protection consists of a resistive
divider network (R3 and R4) as shown in Figure 2. The
resistor divider must be selected so that the voltage at
the OVP pin exceeds the OVP rising threshold when the
output is at V
When the OVP rising threshold is exceeded, the con-
verter stops switching. The open LED channel is removed
from the boost converter feedback loop. When the volt-
age at the OVP pin falls below the OVP hysteresis volt-
age, the boost converter can resume switching.
It is important that during normal operation the current
sinks are given enough headroom that the OVP threshold
is not exceeded.
The output voltage at the minimum OVP threshold is
The maximum voltage of each LED string including the
current sink headroom should not exceed V
1407.2010.06.1.0
SwitchReg
TM
OUT(MAX)
V
6 Channel LED Backlight Driver with Integrated Boost and High Frequency Direct PWM Dimming
OUT(MIN)
V
R3 = R4 ·
RSET
.
= 0.6V
= V
Figure 2: Over-Voltage Protection and Current Sink Setting Circuit.
OVP(MIN)
V
V
OUT(MAX)
OVP(MAX)
·
R
R
- 1
3
4
I
R2
R2
+ 1
V
CC
OUT(MIN)
262 .I
w w w . a n a l o g i c t e c h . c o m
R2
V
.
OUT
Where:
N is the number of LEDs in each string
V
V
V
Another factor in setting the OVP voltage using the resis-
tive divider is that the maximum voltage at the LX pin
should not exceed V
V
V
delay of D1 at turn on.
Measurements should confirm that the maximum switch-
ing node voltage V
case conditions.
OVP(MIN)
CSX
FLED(MAX)
D1
RING
R4
R3
is the forward voltage of the Schottky diode D1
= 0.5V is the current sink voltage
1.2V
is the voltage spike at the LX node caused by the
OVP
= 1.1V is minimum over-voltage threshold
V
R
is the maximum forward LED voltage at 20mA.
OVP(MIN)
3
> R
V
D1
C4
V
LX(MAX)
4
OUT(MIN)
·
·
LX
V
R
R
LX(MAX
LX(MAX)
= V
CSX
3
4
PRODUCT DATASHEET
> V
+ 1
OUT(MAX)
L1
+ N · V
) is less than 50V under worst
CSX
= 50V.
> V
+ N · V
V
AAT1407
FLED(MAX)
OVP(MIN)
+ V
CSX
C1
D1
+ N · V
FLED(MAX)
+ V
VIN
- V
RING
FLED(MAX)
OVP(MIN)
11