AAT1239-1_0810 Datasheet, PDF (11/23 Pages) Advanced Analogic Technologies – 40V Step-Up Converter for 4 to 10 White LEDs

SwitchReg^TM
PRODUCT DATASHEET
AAT1239-1
40V Step-Up Converter for 4 to 10 White LEDs
Application Information
Over-Voltage Protection
OVP Protection with Open Circuit Failure
The OVP protection circuit consists of a resistor network
tied from the output voltage to the OVP pin (see Figure
1). To protect the device from open circuit failure, the
resistor divider can be selected such that the over-volt-
age threshold occurs prior to the output reaching 40V
(V_OUT(MAX)). The value of R3 should be selected from 10kΩ
to 20kΩ to minimize losses without degrading noise
immunity.
R₂
=
R₃
·
⎛
⎝
V_OUT(MAX)
V_OVP
-
1^⎞⎠
AAT1239-1
OVP
GND
VOUT
R2
R3
C_OUT
Figure 1: Over-Voltage Protection Circuit.
1.238V
1.142V
40
30
4
2
0
Time (4ms/div)
Figure 2: Over-Voltage Protection
Open Circuit Response (No LED).
Assume R3 = 12kΩ and V_OUT(MAX) = 40V. Selecting 1%
resistor for high accuracy, this results in R2 = 374kΩ
(rounded to the nearest standard value). The minimum
OVP threshold can be calculated:
V = V _{OUT(OVP_MIN)}
· _OVP(MIN)
⎛
⎝
R₂
R₃
+ 1^⎞⎠
= 35.4V
To avoid OVP detection and subsequent reduction in the
programmed output current (see following section), the
maximum operating voltage should not exceed the
minimum OVP set point.
V < V _OUT(MAX)
OUT(OVP_MIN)
In some cases, this may disallow configurations with
high LED forward voltage (V_FLED) and/or greater than ten
series white LEDs. V_FLED unit-to-unit tolerance can be as
high as +15% of nominal for white LED devices.
OVP Constant Voltage Operation
Under closed loop constant current conditions, the out-
put voltage is determined by the operating current, LED
forward voltage characteristics (V_FLED), quantity of series
connected LEDs (N), and the feedback pin voltage (V_FB).
V_OUT = V_FB + N · V_FLED
When the rising OVP threshold is exceeded, switching is
stopped and the output voltage decays. Switching auto-
matically restarts when the output drops below the
lower OVP hysteresis voltage (100mV typical) and, as a
result, the output voltage increases. The cycle repeats,
maintaining an average DC output voltage proportional
to the average of the rising and falling OVP levels (mul-
tiplied by the resistor divider scaling factor). High oper-
ating frequency and small output voltage ripple ensure
DC current and negligible flicker in the LED string(s).
The waveform in Figure 3 shows the output voltage and
LED current at cold temperature with a ten series white
LED string and V_OVP = 40V. As shown, the output voltage
rises as a result of the increased V_FLED which triggers the
OVP constant voltage operation. Self heating of the
LEDs triggers a smooth transition back to constant cur-
rent control.
1239-1.2008.10.1.2
www.analogictech.com
11