Fixed Frequency white LED Driver
AN1588
General Description
The AN1588 is a step-up DC/DC converter
designed for driving up to 4 white LEDs in
series (Note 4)from a single Lithium Ion battery
Features
cell. AN1588 use current mode, fixed frequency
to regulate the LED current. It’s low 104mV
feedback voltage reduces the power loss and
improves efficiency. the converter will be turn
3.
4.
5.
6.
off if an over voltage condition is present due
to an open circuit condition.
AN1588 built-in under-voltage lockout, current
7. Soft start/PWM dimming
8. UVLO, Thermal shut-down
limiting and thermal overload protection.
Up to 87% efficiency
Up to 1.5Mhz fixed switching frequency
Open load detection and auto shut-down
Low 104mV voltage detect
Application
1.
2.
3.
4.
5.
Order Information
SOT23-5
AN1588F
SOT23-6
1. On-Board MOSFET
2. Drives up to 4 white LEDs
AN1588S
Figure 1: Typical Application Circuit
Cell Phones
Handheld Electronic Device
Digital Still Cameras
Small LCD Display
OLED MP3
Figure 2: Efficiency V.S. LED current



1
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Absolute Maximum Ratings(Note1)
SW
-0.5V to 16V
All Other Pins
-0.3V to 6.5V
Recommended Operating Conditions
(Note2)
Vin
2.5V to 6V
Storage Temp.
Output Voltage
Vin to 16V
Operating Temp
-40℃ to +85℃
Thermal Resistance:
SOT23-5
220℃/W
-55℃ to 150℃
Electrical Characteristics (VIN=VEN=5V, Ta=25℃)
Parameters
Symbol
Operating Input Voltage
VIN
Condition
Min
Typ
2.5
Max
Units
6
V
Supply Current(Shutdown)
VEN=0V
0.1
1
uA
Supply Current(Quiescent)
VFB=0.15V
690
750
uA
1.0
1.3
1.5
Mhz
85
92
Switching Frequency
fSW
Maximum Duty Cycle
IN Under Voltage Lockout
Under Voltage Lockout Hysteresis
Open Lamp Shutdown Threshold
EN Threshold
VFB=0V
UVLO
VIN Rising
VOV
VOV Rising
VEN Rising
VEN Rising
VIN=2.5V
EN Threshold
2.25
1.0
V
1.6
mV
V
V
V
90
EN Input Bias Current
VEN=0V,5V
FB Voltage
FB Input Bias Current
VFB=0.1V
mV
1
uA
114
mV
94
104
-600
-300
nA
0.5
Ohm
350
mA
160
℃
RON
SW Current Limit
Duty Cycle=60%
Thermal Shutdown(Note3)
Notes:
2.45
0.8
EN Hysteresis
SW On-Resistance(Note3)
92
16
1.35
%
1. Exceeding these ratings may damage the device
2. The device is not guaranteed to function outside of its operating rating
3. Guaranteed by Design
4. No guarantee for driving 4 LEDs in series if LED’s Vf > 3.5V due to Vin power
start-up may cause Vout overshoot to 16V that turns on internal OVP function.
2
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Pin Description
SOT26
SOT25
Table1: Pin Assignment
SOT25
SOT26
Pin Name
Pin Function
Power Switch Output. SW is the drain of the internal MOSFET
1
1
SW
switch. By connecting power inductor and output rectifier to SW,
SW can swing between GND and 25V
2
2
GND
Ground
Feedback Input. AN1588 regulates the voltage across the current
3
3
FB
sense resistor between FB and GND. Connect a current sense
resistor from the bottom of the LED string to GND, connect the
bottom of the LED string to FB. The regulation voltage is 104mV.
Regulator On/Off Control Input. EN=High will turn on the
4
4
EN
converter, EN=Low will turn it off. When not used, connect EN to
the input source for automatic startup. The EN pin can not be left
floating.
5
5
OV
6
IN
Over Voltage input. OV measure output voltage for open circuit
protection.
Input Supply Pin. Must be locally bypassed
3
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Functional Description
AN1588 use a fixed frequency, current mode boost regulator to regulate strings of white LEDs. At
the start of each oscillator cycle, the FET is turned on through the control circuitry. A stabilizing
ramp voltage is applied to the output of current sense amplifier the result is fed into the PWM
comparator to prevent sub-harmonic oscillations at duty cycle greater than 50 percent. When this
voltage equals the output voltage of the error amplifier the power FET is turned off. The voltage at
the output of error amplifier is the amplified version of the difference between 104mV and feedback
voltage. If feedback voltage starts to drop the output of the error amplifier increase, this result in
more current to flow through the power FET, thus increasing the power delivered to the output.
Block Diagram
Figure 3: AN1588 Block Diagram
4
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Applications
Figure 4: Driving 3 White LEDs
A typical application circuit can be seen in Figure 4. The 3 white LEDs
can be driven from a 2.5V to 6V at an output current of 20mA. A 0.22uF
output capacitor is sufficient for most applications but an output capacitor up to 1uF may be used. A
10uH inductor with low DCR(Inductor resistance) is recommended to improve efficiency. A 1.0uF
ceramic capacitor is recommended for input capacitance in the real system. Schottky diodes has
fast recovery and low forward voltage and are recommended. Schottky diodes rated 100mA to
200mA are sufficient for AN1588.
The switching characteristics during normal operation can be seen in Figure 5. AN1588 has internal
soft start to limit the current through VIN at start up and also limit the overshoot at the output.
PART NUMBER
MANUFACTURER
URL
CDRH3D16
Sumida or Murata(LQH3C)
www.sumida.com
CMDSH-3
Central Semiconductor Corp.
www.centralsemi.com
C1 MLCC Ceramic
Kyocera
www.kyocera.com
5
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Figure 5: Steady-State Operation
Vin=3.6V, 3 LEDs, 20mA, CH1: SW
CH2: Output Ripple(AC coupled)
CH4: Inductor Current( 10mA/div)
Figure 6: Start-up Waveforms
Vin=3.6V, 3 LEDs, 20mA, CH1: SW
CH2: Internal OV, CH4: Input Current( 100mA/div)
Figure 6 shows the startup behavior of the AN1588. The ramped voltage that is added to the current
sense amplifier reduces the current output as the duty cycle increases. As more LEDs are added, the
output voltage rises but the current delivered to the load is reduced as well.
6
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Setting the LED current

The LED current is set by the feedback resistor R1 in figure 4. The current through the LEDs is
given by the equation 104mV/R1. Table 2 shows the selection of resistors for a desired LED current.
Table 2: ILED V.S. R1
ILED (mA)
R1(Ohm)
1
104
5
20.8
10
10.4
15
6.93
20
5.2
Analog and Digital Dimming
There are three different methods to control dimming of the AN1588 during normal operation. The
first method uses a DC voltage to control the feedback voltage as shown in figure 7. As DC voltage
increases current starts flowing through R1, R2 and R3. The loop will continue to regulate the
feedback voltage to 104mV. Thus the current has to decrease through the LEDs by the same amount
of current from the DC
source. With a Vdc from 0V to 2V, the resistor R2 and R3 can control the LED current from 0mA to
20mA.
Figure 7: Dimming Control by a DC Voltage
7
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
The second method uses a logic signal to do the dimming. Shown in Figure 8. A PWM signal is
applied to the EN pin of AN1588. The LEDs will switch between full load to completely shut off.
The average current through the LEDs will increase proportionally to the duty cycle of the PWM
signal. The PWM signal should be 1KHz or below due to the soft start(150us) function. If the PWM
signal is above 1KHz, dimming can be achieved by using the circuit shown in Figure 10.


Figure 8: PWM Dimming Control
Figure 9: Filtered PWM Dimming Control

8
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Vout = (R1+R2)/R2 x 0.104V
Figure 10: Typical OLED Bias Application 


Figure 11: Over Voltage Protection for AN1588S
9
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Open Load Protection

Built-in open load protection circuit will shut off AN1588 if the output voltage goes too high
(Above 16V). In some case an LED may fail this will result in the feedback voltage always being
zero. This will cause AN1588 run at maximum duty cycle boosting the output voltage higher and
higher, the built-in open load protection circuit will check this conditions and if output exceeds 16V
the AN1588 will shut down.
AN1588 will not operate until the power is turn on again. Figure 11 shows the behavior of AN1588
open load waveform.
Figure 11: Startup Waveforms to an Open Load
Vin=3.6V, CH1:EN, CH2: Built-in OV, CH4: SW
Layout Considerations

Proper PCB layout and component placement will be critical for the high switching frequency to
prevent noise and electromagnetic interference problems. The length and area of all traces connected
to the switch node should be minimized, all components must be closed to IC on PCB to prevent
potential inductance induced.
10
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
PCB Layout Guide (Reference Only)
11
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Package Outline (SOT25)
Symbol Minimum Nominal Maximum
A
0.8
0.95
1.1
A1
0.04
---
0.16
A2
0.7
0.8
0.9
b
0.35
0.4
0.5
c
0.1
0.15
0.25
D
2.7
2.9
3.1
E
1.5
1.7
1.8
e
---
1.9
---
e1
---
0.95
---
H
2.7
2.85
3.0
L
0.35
---
0.55
N
1
5
9

12
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.
Fixed Frequency white LED Driver
AN1588
Package Outline (SOT26)
Symbol Minimum Nominal Maximum
A
0.8
0.95
1.1
A1
0.04
---
0.16
A2
0.7
0.8
0.9
b
0.35
0.4
0.5
c
0.1
0.15
0.25
D
2.7
2.9
3.1
E
1.5
1.7
1.8
e
---
1.9
---
e1
---
0.95
---
H
2.7
2.85
3.0
L
0.35
---
0.55
N
1
5
9
13
Rev. 1.0
Chiplus reserves the right to change product or specification without notice.