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| 型号： | AN-40 |
| 厂商： | Micrel Semiconductor,Inc. |
| 英文描述： | Application Note 40 |
| 中文描述： | 应用笔记40 |
| 文件页数： | 2/3页 |
| 文件大小： | 37K |
| 代理商： | AN-40 |
Application Note 40
Micrel
Application Note 40
2
August 2002
80
81
82
83
84
85
0
5
10
15
20
E
LED CURRENT (mA)
NP04S B220M
CLQ4D10-220
A914BYW-220M
LDR655312T-220W
LQH32CN220K1
VIN = 3.0V
3 White LEDs
L = 22
μ
H
Figure 2. Typical Efficiency vs. LED current
Figure 2 shows the efficiency of the constant-current LED
driver for various inductor manufacturers. With a constant-
current LED driver, efficiencies above 80% can be achieved
with low profile inductor and capacitors.
Parallel Configuration
Figure 3 shows the parallel LED configuration with the
MIC2145 as the LED driver. Similar to the series configura-
tion, the feedback voltage of the MIC2145 and R3 set the
current for one of the white LEDs. By using the same value
resistor for each of the white LEDs, the rest of the white LEDs
should have similar current. The current for each LED will not
match perfectly due to the resistor tolerance, so there will be
a tiny variation. This variation may cause the LED's bright-
ness to be uneven as compared to the series configuration.
In shutdown mode, due to the nature of the boost architec-
ture, the output voltage will equal the input voltage minus the
voltage drop of the Schottky diode. This will cause the white
LEDs to slightly turn on if the voltage of the battery is high
enough. A small switch between the battery and VIN can be
used to disconnect the white LEDs from the battery.
The advantage of the parallel configuration is that the MIC2145
only has to boost the output voltage up to one forward drop
plus the feedback voltage. This allows the MIC2145 to drive
at least 10 white LEDs in parallel with efficiency above 80%
using low profile components.
Brightness Control
The brightness of the LEDs can be easily controlled by
injecting a PWM (Pulse Width Modulation) signal into the
enable pin of the regulator, using a logic control, or applying
a DC voltage from the DAC. Figure 4 shows a 400Hz PWM
signal can be used to control the LEDs
’
current by varying the
duty cycle of the PWM signal. Figure 5 shows a logic signal
and a transistor can be used to switch the LEDs current from
8mA to 1mA and vice versa. Figure 6 shows a DAC can be
used to modulate the LED's current.
L1
10
μ
H
Sumida
CR43-100
MIC2145
D1
On Semiconductor
MBR0530T1
RSET
EN
VDD
SW
R3
V
5V/150mA
PGND
FB
SGND
R1
100k
GND
JP1
+V
3.0V to 4.2V
SS
C2
0.01
μ
F
C1
10
μ
F/6.3V
Murata
GRM40 X5R 106K 6.3
R2
10k
R3
C3
10
μ
F/6.3V
Murata
GRM40 X5R 106K 6.3
LED1
LED10
Figure 3. Parallel Configuration
L1
22
μ
H
Murata
LQH32CN220K1
U1
MIC2142
EN
SW
VCC
GND
FB
1
2
3
4
5
D1
MBR0530
R1
86.6
C3
10
μ
F/16V
Murata
GRM42-6X5R106K16
C2
1
μ
F
25V
+VIN
3.0V to 4.2V
C1
10
μ
F/6.3V
Taiyo Yuden
JMK212BJ106MG
GND
PWM
Z1
16V
Blue LEDs
Figure 4. Series White LED Driver with PWM Brightness Control
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