AAT1239-1_0810 Datasheet, PDF (12/23 Pages) Advanced Analogic Technologies – 40V Step-Up Converter for 4 to 10 White LEDs

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PRODUCT DATASHEET
AAT1239-1
40V Step-Up Converter for 4 to 10 White LEDs
V_OUT
(5V/div)
Over-Voltage Protection
Cold Temperature Apply Self-Recovery
I_LED
(200mA/div)
where:
V_FB(MAX) = 0.4V when SEL = Low
V_FB(MAX) = 0.6V when SEL = High
i.e., for a maximum LED current of 20mA (SEL = High):
R_BALLAST =
V_FB
I_LED(MAX)
=
0.6
0.020
=
30Ω
≈
30.1Ω
Figure 3: Over-Voltage Protection
Constant Voltage Operation
(10 White LEDs; I_LED = 20mA;
R₂ = 12kΩ; R₃ = 374kΩ).
While OVP is active, the maximum LED current program-
ming error (ΔI_LED) is proportional to voltage error across
an individual LED (ΔV_FLED).
ΔV_FLED =
(N · V_FLED(TYP) - V_{OUT(OVP_MIN)} - V_FB)
N
To minimize the ΔI_LED error, the minimum OVP voltage
(V_{OUT(OVP_MIN)}) may be increased, yielding a corresponding
increase in the maximum OVP voltage (V_{OUT(OVP_MAX)}).
Measurements should confirm that the maximum switch-
ing node voltage (V_SW(MAX)) is less than 45V under worst-
case operating conditions.
V_SW(MAX) =
V_OVP(MAX) ·
⎛
⎝
R₃
R₂
+ 1^⎞⎠ + V_F + V_RING
V_F = -Schottky Diode DS1 forward voltage at turn-OFF
V_RING = Voltage ring occurring at turn-OFF
LED Selection and Current Setting
The AAT1239-1 is well suited for driving white LEDs with
constant current. Applications include main and sub-LCD
display backlighting, and color LEDs.
The LED current is controlled by the FB voltage and the
ballast resistor. For maximum accuracy, a 1% tolerance
resistor is recommended.
The ballast resistor (R_BALLAST) value can be calculated as
follows:
R = ^VI _BALLAST
FB(MAX)
LED(MAX)
Maximum I_LED
Current (mA)
30
25
20
15
10
5
R_BALLAST (Ω)
SEL = High SEL = Low
20.0
24.3
30.1
40.2
60.4
121.0
13.3
16.2
20.0
26.7
40.2
80.6
Table 1: Maximum LED Current and R_BALLAST
Resistor Values (1% Resistor Tolerance).
Typical white LEDs are driven at maximum continuous
currents of 15mA to 20mA. The maximum number of
series connected LEDs is determined by the minimum
OVP voltage of the boost converter (V_{OUT(OVP_MIN)}), minus
the maximum feedback voltage (V_FB(MAX)) divided by the
maximum LED forward voltage (V_FLED(MAX)). V_FLED(MAX) can
be estimated from the manufacturers’ datasheet at the
maximum LED operating current.
V = V _{OUT(OVP_MIN)}
· _OVP(TYP)
⎛
⎝
R₂
R₃
+ 1^⎞⎠
N=
(V_{OUT(OVP_MIN)} - V_FB(MAX))
V_FLED(MAX)
Figure 4 shows the schematic of using ten LEDs in series.
Assume V_FLED @ 20mA = 3.5V (typical) from LW M673
(OSRAM) datasheet.
V_{OUT(OVP_MIN)} = 1.2V ·
⎛
⎝
374kΩ
10.4kΩ
+
1^⎞⎠
= 38.6V
N
=
38.6V - 0.6V
3.5V
≈ 10.9
Therefore, under these typical operating conditions, ten
LEDs can be used in series.
12
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1239-1.2008.10.1.2