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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
Introduction
The AAT1239-1 is a varied frequency, constant current boost converter capable of driving up to 20 white LEDs. This
paper discusses how to select external components for the AAT1239-1 and enhance its efficiency based on power loss
analysis in Li-ion battery applications.
AAT1239-1 Typical Application Connection
Figure 1 shows a typical application connection for the AAT1239-1. The AAT1239-1 can drive up to 20 WLEDs (10 in
series, and 2 branches).
S2C
High or low level
1
VIN
3.0V~5.5V
U1
AAT1239-1
PVIN
EN/SET
3
SEL
2
C1
4
5
6
L1
12
LIN
11
OVP
10
FB
9
VIN
AGND
N/C
SW
PGND
7
SW
8
DS1
SS16L
IOUT
R2
C2
R3
12k
VOUT
D11
D21
D12
D22
D13
D23
D1x
D2x
R1
Figure 1: AAT1239-1 Typical Application Connection.
Power Loss Analysis
Several factors influence the AAT1239-1's efficiency, including switching loss, PMOSFET disconnect power loss, inductor
copper loss, control circuit power consumption, and external resistor power loss. Each factor will be analyzed and calculated in the following sections.
PMOSFET Power Loss
The AAT1239-1 has a P-channel MOSFET connected between PVIN and LIN. This P-channel MOSFET senses input current. Power loss on the P-channel MOSFET is determined by RDS(ON) current level. Figure 2 shows the power loss of the
P-channel MOSFET vs. current level. The curve shows that the power loss is less than 25mW when the current level is
less than 30mA.
Table 1 shows the AAT1239-1’s input current and PMOSFET power loss with a 2.2µH inductor driving 10 WLEDs in series
(31V VOUT) with 20mA LED current. The table shows that the PMOSFET power loss is only 1.5%. This has little effect on
the efficiency of the application.
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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
100
Power Loss (mW)
90
80
70
60
50
40
30
VIN = 3.0V
VIN = 3.6V
VIN = 4.2V
20
10
0
0
50
100
150
200
250
300
350
400
450
500
550
600
Current (mA)
Figure 2: AAT1239-1 PMOSFET Power Loss vs. Current.
Measurement Data
Calculation Result
L (µH)
VIN (V)
IIN (mA)
VOUT (V)
IOUT (mA)
RDS(ON) (mΩ)
PIN (mW)
PLOSS (mW)
PLOSS/PIN (%)
2.2
3.61
251
31.2
20.8
211
906
13.3
1.5
Table 1: AAT1239-1 PMOSFET Power Loss with 2.2µH Inductor.
Inductor Copper Loss
Inductor copper power loss is determined by the RMS current flowing through the inductor and the inductor DCR value.
Inductor RMS current is a root-mean-square value of inductor current which includes DC and AC factors. Inductance
directly affects inductor AC current, also called ripple current. The ripple current in Continuous Conduction Mode from
a boost converter can be derived from the following formula:
∆IINDUCTOR_PP =
VIN · D
f·L
Higher inductance creates a lower ripple current; lower inductance creates a higher ripple current.
Figure 3 shows a AAT1239-1 operating waveform at 3.6V VIN, 31V VOUT (10 white LEDs in series), 20mA LED current
with 2.2µH and 10µH inductors with similar DCR (48mΩ). Differences in inductor RMS current can be observed at varying inductance, leading to differences in inductor copper loss.
For example, with a 2.2µH (CDRH2D18/HP-2R2) inductor:
PIN = VIN · IIN = 3.61 · 251 = 906mW
PLOSS_L = I2L_RMS · DCR = (336mA)2 · 48mΩ = 5.4mW
PLOSS_L
5.4
=
· 100% = 0.6%
PIN
906
Table 2 illustrates inductor copper loss with 2.2µH and 10µH inductors, 10 WLEDs in series (31V VOUT), and 20mA LED
current. In the AAT1239-1 application, inductor copper loss leads to approximately 2% power loss.
2
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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
(a) 2.2µH inductor: CDRH2D18/HP-2R2
(b) 10µH inductor: CDRH5D28-100
Figure 3: AAT1239-1 Operating Waveform with Different Inductors at 3.6V VIN, 10 WLEDs in series
(VOUT = 31V), 20mA LED Current; Ch1: SW, Ch3: IINDUCTOR.
Inductor
Measurement Data
Calculation Result
Inductance
(µH)
Part Number
DCR (mΩ)
VIN (V)
IIN
(mA)
IL_RMS
(mA)
PIN
(mW)
PLOSS
(mW)
PLOSS/PIN
(%)
2.2
10
10
CDRH2D18/HP-2R2
CDRH5D28-100
CDRH3D18-100
48
48
164
3.61
3.61
3.61
251
227
223
336
287
282
906
819
805
5.4
4.0
13.0
0.6
0.5
1.6
Table 2: AAT1239-1 Inductor Copper Loss at Different Inductor Values.
Switching Loss
Switching transitions of the power MOSFET can lead to a very large instantaneous power loss. Even though transition
times are short, the resulting average power loss is significant. The average switching loss is positively proportional to
the switching frequency, switching transition time, etc.
The switching frequency of the AAT1239-1 varies with different conditions (VIN, VOUT, IOUT and L) according to its constant current control structure. This leads to different switching losses with varying inductor values. A large switching
frequency difference is illustrated in Figure 3. As a result, there is an 87mW difference in switching loss, which is 9.6%
of the total power at 3V VIN as shown in Table 3.
Measurement Data
Calculation Result
L (µH)
VIN (V)
IIN (mA)
VOUT (V)
IOUT (mA)
Freq. (MHz)
PIN (mW)
POUT (mW)
2.2
10
2.2
10
3.0
3.0
4.2
4.2
300
271
216
196
31.4
31.4
31.4
31.4
20.8
20.8
20.9
20.8
1.49
0.58
1.87
0.94
900
813
907
823
653
653
654
654
P∆ (mW)
P∆/PIN
(%)
87
9.6
84
9.3
Table 3: Power Loss Difference Mainly Caused by Switching Loss.
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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
The switching frequency of the AAT1239-1 increases as input voltage increases and decreases as inductance increases.
Figure 4 shows at VIN = 3.0V, different inductance, frequency vs. IOUT curve. The curve shows that the AAT1239-1's
switching frequency is 1.49MHz for 2.2µH, 0.86MHz for 4.7µH, and 0.58MHz for 10µH at 3.0V VIN. The frequency change
is almost 2.6 times greater when the inductor is changed from 2.2µH to 10µH. Paired with Table 3, this tells us that a
10µH inductor can decrease the switching loss by 9% compared to a 2.2µH inductor. 1.8
L = 2.2µH
L = 4.7µH
L = 10µH
Frequency (MHz)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
70
80
90
IOUT (mA)
Figure 4: AAT1239-1 Frequency vs. IOUT at VIN = 3.0V.
Control Circuit and External Resistor Power Loss
The quiescent current of AAT1239-1 is only 5µA at 3.6V VIN when SEL and EN = logic high, VFB = 0.6V and no LED load.
At this instant, the AAT1239-1 internal control circuit only consumes 0.02mW.
External power consumption is from the OVP divider (R2 and R3) and the ballast resistor (R1). Even with a load of 10
WLEDs in series (VOUT voltage is assumed to be 31V), the OVP resistor divider power consumption is 2.5mW which is
only 0.3% of the total 906mW at 3.6V VIN.
Table 4 gives a summary of the above analysis and illustrates that the switching loss is the most significant power loss.
L (µH)
PMOSFET Power
Loss (%)
Inductor Copper
Loss (%)
Switching Loss
(%)
Control Circuit and External
Resistor Power Loss (%)
2.2
10
1.5
1.4
2
2
24
12
0.3
0.3
Table 4: AAT1239-1 Power Loss Summary at 3.6V VIN, 10 WLEDs in Series, 20mA LED Current.
Application Solution with Enhanced Efficiency
From the efficiency analysis above, decreasing the switching loss can remarkably improve efficiency by increasing the
inductance of the inductor. Inductance of the inductor can not be selected randomly because the designer has to consider the loop stability and application circuit size. Proper inductor selection is important in the actual application. Under
sizing or over sizing inductance may cause instability to the AAT1239-1’s control loop which might generate a greater
output ripple (above 500mV) and cause the inductor to vibrate. A bigger inductor value with higher saturation current
might lead to the increase of the total application solution (circuitry size).
4
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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
Inductor and Capacitor Selection
In inductor selection, inductance and saturation current are the most important two factors. The principle of inductor
selection for AAT1239-1 is that the IC should produce stable LED current under whole application operating range. Table
5 gives a selection guide of inductance, output capacitance and loop stable area when AAT12391-1 drives up to 10
WLEDs in Li+ battery application. For example, in the application of 10S2P (10 WLEDs in series and 2 in parallel) with
20mA LED current in each string, both a 2.2µH and a 4.7µH inductance can be used. However, the combination of a
4.7µH inductor and a 2.2µF output capacitor is more efficient.
Loop Stable Area
L (µH)
COUT
IOUT_MAX (mA)
Operating Range (V)
2.2
4.7
10
10
2.2µF/50V
2.2µF/50V
2.2µF/50V
4.7µF/50V
60
40
25
30
3.0 ~ 5.5
Table 5: AAT1239-1 Stable Area with Inductor and Output Capacitor Combination.
Inductor saturation current is another important factor. The saturation current specified in the inductor datasheet represents the current when inductance become 35% lower than its nominal value.
Under a basic boost structure, in CCM (Continuous Current Mode), the inductor peak current value is determined by
the following formula:
IINDUCTOR_PEAK =
ILOAD
V
1
+ IN · D ·
1·D 2·L
f
D=1-
VIN
VOUT
This formula is also suitable for AAT1239-1. The device's frequency varies by up to 2MHz when VIN, IOUT and L change.
Though we can calculate inductor peak current value based on the formula, it is difficult to derive the value because
there is no formula to estimate the frequency. Figure 5 illustrates inductor peak current value vs. output current at
3.0V VIN. 3.0V VIN is the least suitable input voltage condition in most applications. The customer can select the minimum inductor saturation current based on Figure 5. For example, when IOUT is 40mA, saturation current for a 4.7µH
inductor should be higher than 1.1A.
IINDUCTOR_MAX (mA)
2.00
1.75
1.50
1.25
1.00
0.75
L = 2.2µH
L = 4.7µH
L = 10µH
0.50
0.25
0.00
10
20
30
40
50
60
IOUT (mA)
Figure 5: AAT1239-1 Inductor Peak Current Value Under Different Inductance at VIN = 3.0V.
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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
Resistor Selection
The OVP resistor divider (R2 and R3) threshold is used to detect the trip voltage if one of the LEDs in the string is
disconnected. The output voltage is limited and will recover once the open connection condition is removed.
Recommended OVP resistor selection when R1 is equal to 12kΩ is shown in Table 6.
Number of LED (in series)
R2 (kΩ)
OVP Trip Point (V)
4
5
6
7
8
9
10
158
182
215
255
287
324
374
15.8
18.2
21.5
25.5
28.7
32.4
37.4
Table 6: AAT1239-1 OVP Resistor Selection.
Resistor R1 is used to program the LED current. The value of R1 can be calculated using the following equation:
R1 =
0.6V
ILED
Table 7 shows several resistor values corresponding to the most commonly used LED current levels.
R1 (Ω)
Maximum ILED (mA)
SEL = High
SEL = Low
40
35
30
25
20
15
10
5
15.0
16.9
20.0
24.3
30.1
40.2
60.4
121.0
10.0
11.3
13.3
16.2
20.0
26.7
40.2
80.6
Table 7: Maximum LED Current and R1 Resistor Values (1% Resistor Tolerance).
Application Solution Example
Table 8 illustrates two application examples of external component selection. The key factor is the inductor. As shown
in Table 5, a 10µH inductor can be used for a 10S/20mA application (10 WLEDs in series and 20mA LED current).
According to Figure 5, the 10µH inductor should have greater than 0.6A saturation current for the full scale LED current
(20mA). The Sumida CDRH3D18-100NC SMD inductor is suitable for the application and has a 0.9A saturation current
and an inductor size of 4.0x4.0x2.0mm.
6
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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
Application: WLED/IOUT(MAX)
Component
C1
L1
C2
R1 (Ω)
R2 (kΩ)
10S/20mA
10S2P/40mA
4.7µF/10V/0805
GRM219R61A475KE19
10µH
CDRH3D18-100NC
2.2µF/50V/1206
GRM31CR71H225KA88
30.1
374
4.7µF/10V/0805
GRM219R61A475KE19
4.7µH
CDRH4D22/HP-4R7
2.2µF/50V/1206
GRM31CR71H225KA88
15
374
90.0
85.0
87.5
82.5
85.0
80.0
82.5
80.0
77.5
VIN = 3.0V
VIN = 3.6V
VIN = 4.2V
75.0
72.5
70.0
2
4
Efficiency (%)
Efficiency (%)
Table 8: Application Solution Example.
6
8
10
12
14
16
18
Output Current (mA)
(a) 10S/20mA Solution Efficiency
77.5
75.0
72.5
VIN = 3.0V
VIN = 3.6V
VIN = 4.2V
70.0
67.5
65.0
20
5
10
15
20
25
30
35
40
Output Current (mA)
(b) 10S2P/40mA Solution Efficiency
Figure 6: Application Solution Efficiency.
Summary
Power loss sources in the AAT1239-1 include switching loss, PMOSFET disconnect power loss, inductor copper loss,
control circuit power consumption, and external resistor power loss. Among these, switching loss is the highest power
loss source.
Increasing inductance can significantly improve the efficiency of the AAT1239-1 by decreasing the switching frequency,
which reduces switching loss. However, large inductance can make the AAT1239-1 unstable. Designers should select
the correct inductance for their specific application according to Table 5. It is also important to choose an inductor with
sufficient inductor saturation current as shown in Figure 5.
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APPLICATION NOTE
AAT1239-1 Enhanced Efficiency Application Solution
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202371A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012