PRODUCT DATASHEET
AAT1239-1
SwitchRegTM
40V Step-Up Converter for 4 to 10 White LEDs
where:
Over-Voltage Protection
Cold Temperature Apply Self-Recovery
VFB(MAX) = 0.4V when SEL = Low
VFB(MAX) = 0.6V when SEL = High
VOUT
(5V/div)
i.e., for a maximum LED current of 20mA (SEL = High):
ILED
(200mA/div)
VFB
ILED(MAX)
0.6
0.020
≈
RBALLAST
=
=
= 30Ω 30.1Ω
Figure 3: Over-Voltage Protection
Constant Voltage Operation
(10 White LEDs; ILED = 20mA;
R2 = 12kΩ; R3 = 374kΩ).
Maximum ILED
Current (mA)
RBALLAST (Ω)
SEL = High SEL = Low
30
25
20
15
10
5
20.0
24.3
30.1
40.2
60.4
121.0
13.3
16.2
20.0
26.7
40.2
80.6
While OVP is active, the maximum LED current program-
ming error (ΔILED) is proportional to voltage error across
an individual LED (ΔVFLED).
Table 1: Maximum LED Current and RBALLAST
Resistor Values (1% Resistor Tolerance).
(N · VFLED(TYP) - VOUT(OVP MIN) - VFB)
_
ΔVFLED
=
N
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 (VOUT(OVP_MIN)), minus
the maximum feedback voltage (VFB(MAX)) divided by the
maximum LED forward voltage (VFLED(MAX)). VFLED(MAX) can
be estimated from the manufacturers’ datasheet at the
maximum LED operating current.
To minimize the ΔILED error, the minimum OVP voltage
(VOUT(OVP_MIN)) may be increased, yielding a corresponding
increase in the maximum OVP voltage (VOUT(OVP_MAX)).
Measurements should confirm that the maximum switch-
ing node voltage (VSW(MAX)) is less than 45V under worst-
case operating conditions.
⎛ R3
⎝ R2
⎞
⎛ R2
⎝ R3
⎞
+ 1
·
+ 1 + VF + VRING
VSW(MAX) = VOVP(MAX)
·
VOUT(OVP_MIN) = VOVP(TYP)
⎠
⎠
(VOUT(OVP_MIN) - VFB(MAX)
)
N =
VF = -Schottky Diode DS1 forward voltage at turn-OFF
VRING = Voltage ring occurring at turn-OFF
VFLED(MAX)
Figure 4 shows the schematic of using ten LEDs in series.
Assume VFLED @ 20mA = 3.5V (typical) from LW M673
(OSRAM) datasheet.
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.
⎛ 374kΩ
⎝10.4kΩ
⎞
= 38.6V
·
+ 1
VOUT(OVP_MIN) = 1.2V
⎠
38.6V - 0.6V
The LED current is controlled by the FB voltage and the
ballast resistor. For maximum accuracy, a 1% tolerance
resistor is recommended.
N =
3.5V
≈ 10.9
The ballast resistor (RBALLAST) value can be calculated as
follows:
Therefore, under these typical operating conditions, ten
LEDs can be used in series.
VFB(MAX)
RBALLAST
=
ILED(MAX)
w w w . a n a l o g i c t e c h . c o m
12
1239-1.2008.10.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
