Diodes AP8800A User Manual

A

Description

The AP8800A is a step-down DC/DC converter designed to drive
LEDs with a constant current. The device can drive up to 7 LEDs,
depending on the forward voltage of the LEDs, in series from a
voltage source of 8V to 28V. Series connection of the LEDs provides
identical LED currents resulting in uniform brightness and eliminating
the need for ballast resistors. The AP8800A switches at frequency up
to 600kHz. This allows the use of small size external components,
hence minimizing the PCB area needed.

Maximum output current of AP8800A is set via an external resistor
connected between the V
by applying either a DC voltage or a PWM signal at the CTRL input
pin. An input voltage of 0.2V or lower at CTRL switches off the output
MOSFET simplifying PWM dimming.

and SET input pins. Dimming is achieved

IN

Features

COST EFFECTIVE 28V BUCK 1-W LED DRIVER

Pin Assignments

SW

GND

CTRL

Applications

(Top View)

1

2

3

5V

IN

4 SET

P8800A

• LED driving current up to 370mA

• Better than 5% accuracy

• High efficiency up to 95%

• Operating input voltage from 8V to 28V

• Simple, versatile solution requiring only 4 components

• PWM/DC input for dimming control

• Built-in output open-circuit protection

• TSOT25: Available in “Green” Molding Compound (No Br, Sb)

with lead Free Finish/ RoHS Compliant

 Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2)
 Halogen and Antimony Free. “Green” Device (Note 3)

Notes: 1. No purposely added lead. Fully EU Directive 2002/95/EC (RoHS) & 2011/65/EU (RoHS 2) compliant.

2. See http://www.diodes.com for more information about Diodes Incorporated’s definitions of Halogen and Antimony free, "Green" and Lead-Free.

3. Halogen and Antimony free "Green” products are defined as those which contain <900ppm bromine, <900ppm chlorine (<1500ppm total Br + Cl)
and <1000ppm antimony compounds.

• MR16 Lamps

• General Illumination Lamps

Typical Applications Circuit

AP8800A

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Pin Descriptions

Pin Name Pin Number Function

SW 1 Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to reduce EMI.

GND 2 GND Pin

Dimming and On/Off Control Input.

• Input voltage of 0.2V or lower forces the device into low current standby mode and shuts off the output.

CTRL 3

SET 4

VIN 5

An open-drain/collector PWM signal allows the output current to be adjusted below the level set by the
resistor connected to SET input pin.

• The input impedance is about 50kΩ, and if the pin is left open V

• Drive with DC voltage (0.3V < V

< 2.5V) to adjust output current from 24% to 200% of I

CTRL

CTRL

= V

REF

.

Set Nominal Output Current Pin.
Configures the output current of the device.

Input Supply Pin. Must be locally decoupled to GND with >

2.2µF X7R ceramic capacitor – see applications

section for more information.

Functional Block Diagram

P8800A

OUTnom

Absolute Maximum Ratings (@T

= +25°C, unless otherwise specified.)

A

Symbol Parameter Ratings Unit

VIN Continuous VIN Pin Voltage Relative to GND -0.3 to +30 V

V

Set Voltage Relative to VIN V

SET

-5 to VIN +0.3 V

IN

VSW SW Voltage Relative to GND -0.3 to +30 V

V

CTRL Pin Input Voltage -0.3 to +5 V

CTRL

ISW Switch Current 0.45 A

TJ Junction Temperature 105 °C

T

Lead Temperature Soldering 300 °C

LEAD

T

Storage Temperature Range -55 to +150 °C

ST

Caution: Stresses greater than the 'Absolute Maximum Ratings' specified above, may cause permanent damage to the device. These are stress ratings only;

Semiconductor devices are ESD sensitive and may be damaged by exposure to ESD events. Suitable ESD precautions should be taken when
handling and transporting these devices.

functional operation of the device at these or any other conditions exceeding those indicated in this specification is not implied. Device reliability may be
affected by exposure to absolute maximum rating conditions for extended periods of time.

AP8800A

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P8800A

Recommended Operating Conditions (@T

Symbol Parameter Min Max Unit

VIN Operating Input Voltage Relative to GND 8.0 28 V

V

Voltage Range for 24% to 200% DC Dimming Relative to GND 0.3 2.5 V

CTRLDC

V

Voltage Low for PWM Dimming Relative to GND 0.2 V

CTRLL

ISW Continuous Average Switch Current (Note 4) 370 mA

D

Duty Cycle Range Applied to CTRL (f < 500Hz) 0.01 1

PWM

fSW Switching Frequency 600 kHz
TA Ambient Temperature Range -40 105 °C

Note: 4. Dependent on junction and ambient temperature.

Electrical Characteristics (@T

Symbol Parameter Conditions Min Typ Max Unit

IQ Quiescent Current V

ISD Shutdown Current V

V

Internal Threshold Voltage 95 100 105 mV

THD

V

Internal Reference Voltage 1.25 V

REF

I

SET Pin Input Current V

SET

R

On Resistance of MOSFET 1.5 2.2 Ω

DS(ON)

I

Switch Leakage Current 5 µA

SW-LKG

fSW Switching Frequency with L = 100µH

tSS Soft-Start Time

tPD Internal Comparator Propagation Delay 55 ns

θ

Thermal Resistance Junction-to-Ambient TSOT25 (Note 5) 200 °C/W

JA

Note: 5. Test condition for TSOT25: Device mounted on FR-4 PCB (25mm x 25mm 1oz copper, minimum recommended pad layout on
top layer and thermal vias to bottom layer ground plane. For better thermal performance, larger copper pad for heat-sink is needed.

= +25°C, VIN = 12V, unless otherwise specified.)

A

= +25°C, unless otherwise specified.)

A

= 1.25V, fSW = 250kHz 300 650 µA

CTRL

= 0V 20 30 µA

CTRL

= VIN-0.1 1.3 µA

SET

ADJ pin floating L = 100H
I

= 350mA @ 1 LED (V

OUT

= 3.4V)

LED

Time taken for output current to reach 90%
of final value after V

CTRL

= 0.3V

250 kHz

500 µs

AP8800A

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0

5

5

5

Typical Characteristics

50

400

T= 25°C

A

2

20

T = 25°C

A

P8800A

300

200

Quiescent Current (µA)

100

0

04 812 16 20 24 28

Input Voltage (V)

Quiescent Current (Switching) vs. Input Voltage

1.242

1.242

Reference Voltage (V)

T = 25°C

A

15

10

Shutdown Current (µA)

5

0

048121620 24 28

Input Voltage (V)

Shutdown Current (Non-Switching) vs. Input Voltage

1.25

1.25

1.245

REF

V (V)

1.24

V = 12V

IN

1.2415
4812 16 20 24 28

Input Voltage (V)

Reference Voltage vs. Input Voltage

1.4

1.2

1

0.8

0.6

Reference Voltage (V)

0.4

T = 25°C

A

1.235

-40 -15 10 35 60 85 1 10
Ambient Temperature ( °C)

Reference Voltage vs. Ambient Temperature

2.4

V = 12V

IN

2.2

2

1.8

1.6

1.4

Power Switch On-Resistance ( )Ω

0.2

0

012345678910

Input Voltage (V)

Reference Voltage vs. Input Voltage (Low Voltage)

1.2

1

-40 -15 10 35 60 85
Ambient Temperature (°C)

Power Switch On-Resistance vs. Ambient Temperature

110

AP8800A

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Typical Characteristics (cont.)

2

V = 12V

IN

L = 100µH

Ω

R = 0.33

S

1

350

300

250

R = 0.3

S

Ω

R = 0. 56

S

P8800A

Ω

0

-1

Deviation From Nominal Set Current(%)

-2

-40 -15 10 35 60 85 110
Ambient Temperature (°C)

Change in LED Current vs. Ambient Temperature

AP8800A

Document number: DS35100 Rev. 3 - 2

200

150

OUT

I Mean (mA)

100

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50

0

00.51

Output Current vs. V

R = 1

Ω

S

1.5 2 2.5

V (V)

CTRL

CTRL

3

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P8800A

Application Information (@T

AP8800A Operation

The AP8800A is a hysteretic LED current switching regulator sometimes known as an equal ripple switching regulator. In normal operation, when
voltage is applied at +V
and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +V

(See Figure 1), the AP8800A internal switch is turned on. Current starts to flow through sense resistor R1, inductor L1,

IN

= +25°C, VIN = 12V, unless otherwise specified.)

A

, and the inductor L1 (See Figure 2).

IN

Figure 1 Typical Configuration

This rising current produces a voltage ramp across R
proportional voltage to the input of the internal comparator.

When this voltage reaches an internally set upper threshold, the internal switch is turned off. The inductor current continues to flow through R
L1, the LEDs and the schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined b y the for ward voltage drop
of the LEDs and the schottky diode.

This decaying current produces a falling voltage at R
R

is applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the internal switch is turned on

SET

again. This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor R
determined by the input voltage and LED chain voltage.

In normal operation the off time is relatively constant (determined mainly by the LED chain voltage) with only the on-time varying as the input
voltage changes. At duty cycles up to around 80% the ramp of the LED/switch current is very linear; however, as the duty cycle approaches 95%
the LED current ramp starts to become more exponential. This has two effects:

1. The overall on time starts to increase lowering the overall switching frequency.

2. The average LED current starts to increase – which may impact accuracy.

. The internal circuit of the AP8800A senses the voltage across R

SET

, which is sensed by the AP8800A. A voltage proportional to the sense voltage across

SET

Figure 2 Typical Switching Waveform

SET

with a switching current

SET,

and applies a

SET

LED Current Control

With the CTRL pin left floating and the external current sense resistor, R
average output current in the LEDs is:

V

I =

LED

TH

where VTH is nominally 100mV

R

SET

(greater than 0.3 Ω) is connected between VIN and SET, the nominal

SET

,

AP8800A

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P8800A

Application Information (cont.) (@T

Inductor Selection

A 68μH inductor or higher, is recommended for most AP8800A applications with input voltage at 12V.
Figure 3 displays the resulting switching frequency with various main circuit parameters: Supply voltage, Inductor value and number of LEDs to

be driven.

700

= +25°C, VIN = 12V, unless otherwise specified.)

A

Switching Frequency

600

500

400

300

Frequency [kHz]

200

100

0

0 50 100 150 200 250 300 350 400 450 500

47

Figure 3 Switching Frequency vs. Supply Voltage, Inductor, and Number of LEDs

100

150

68

220

100

150

220

Induc to r Value [u H]

470

470

12V - 1LED
24V - 3 LEDs
28V - 4LEDs

Capacitor Selection

A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply source impedance and
lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input
supply. A minimum value of 1μF is acceptable if the DC input source is close to the device, but higher values will improve performance at lower
input voltages, especially when the source impedance is high. The input capacitor should be placed as close as possible to the IC.
For AC input sources a bigger capacitor ( 300uF in the case of 12C AC) to guarantee the accuracy of the LED current.

For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended.
Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used.

Diode Selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance SBR diode with low reverse leakage at the
maximum operating voltage and temperature. The recommended diode for use with this part is the SBR1A40, with a current rating above the
peak coil current and a continuous current rating higher than the maximum output load current. It is very important to consider t he reverse
leakage of the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device.

AP8800A

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P8800A

Application Information (cont.) (@T

LED Current Dimming

The LED current can be dimmed in two ways;

1. Analog Dimming: Where a dc voltage is applied to the CTRL pin
or

2. PWM Dimming: Where a Pulse Width Modulated (PWM) signal is applied to the CTRL pin.

Analog Dimming

If the CTRL pin is driven by an external voltage (lower than 2.5V), the average LED current in this case is:

V

I ×=

LED

CTRL

V

A DC signal from 0.3V to 2.5V applied to the CTRL pin will vary the LED current from 24% to 200% of nominal LED current. This gives an
approximate 8:1 dimming range; care, should be exercised when overdriving the CTRL pin to 200% of nominal LED current not to exceed the
power dissipation of the package.

The graph in figure 4 shows values of nominal average output current for 3 values of current setting resistor (R
circuit shown on Figure 1, for different voltages applied on the CTRL pin.

REF

V

TH

R

SET

= +25°C, VIN = 12V, unless otherwise specified.)

A

) in the typical application

SET

400

350

300

250

200

150

LE D Current [mA]

100

50

0

0 0.15 0.3 0.45 0.6 0.75 0.9 1.05 1.2 1.35 1.5 1.65 1.8 1.95 2.1 2.25 2.4 2.55 2.7 2.85 3

0.3

0.3

0.33

0.39

0.39

0.47

0.47

AP8800A

Document number: DS35100 Rev. 3 - 2

0.56

0.56

0.56

0.68

0.68

0.68

LED current Versus RSET and VCTRL

0.75

0.82
1

1.2

0.82
1

1.2

0.82

1

1.2

R SET value [Oh ms]

Figure 4 LED Current Setting vs. R

1.5

1.5

1.5

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LED current @
VCTR L = 1.25V

LED current @
VCTR L = 2.5V

LED current @

2
2

2

and V

SET

CTRL

3
3

3

VCTR L = 0.625V

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P8800A

Application Information (cont.) (@T

PWM Dimming

A PWM signal with a max resolution of 8bit can be applied to CTRL regulate the output current to a value below the nominal average value set
by resistor R
applying dc voltage to the CTRL pin to achieve average LED current dimming.

Figure 5 shows the typical PWM response of the AP8800A. An internal filter produces a rump.

. PWM dimming gives a wider average LED current variation and is more accurate at lower average LED currents than by

SET

= +25°C, VIN = 12V, unless otherwise specified.)

A

Figure 5 Typical PWM Dimming Waveform

The recommended method of driving the CTRL pin and controlling the amplitude of the PWM waveform is to use a small NPN switching
transistor as shown below:

Figure 6 Open Collector PWM Dimming Circuit

This scheme uses the 200k resistor between the ADJ pin and the internal voltage reference as a pull-up resistor for the external transistor eg
MMBT3904.

Soft-Start

An external capacitor from the CTRL pin to ground will provide soft-start delay, by increasing the time taken for the voltage on this pin to rise to
the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator.

The soft-start time is 0.5ms/nF.

AP8800A

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P8800A

Application Information (cont.) (@T

Fault Conditions

The AP8800A is inherently protected against open-LED conditions. If one LED becomes open circuit the device automatically stops switching
and will only retart if the open-LED fault is removed.

If one or more LEDs should become shorted together then the s witching frequency and duty cycle will change. If one or m ore LEDs get shorted
together, the ramp-up time of LED current will become shorter due to there being a larger voltage across the inductor. However, the ramp-down
time of the LED current will increase due to the voltage across the inductor becoming smaller.

Figure 7 below shows the AP8800A driving 3 LEDs when all 3 LEDs become shorted together. Due to the large voltage change across the
inductor during both LED current ramp-up and ramp-down we see a large difference in switching frequency.

= +25°C, VIN = 12V, unless otherwise specified.)

A

Figure 7 LED Short Fault Condition

Thermal Considerations

The graph below in Figure 8, gives details for the power derating of the AP8800AWT. This assumes the device to be mounted on a 25 x 25mm
PCB with 1oz copper standing in still air.

450

400

350

300

250

200

150

Powe r d issi p ation (mW)

100

50

0

-40-25-10 5 203550658095110

Ambient temperature (°C)

Figure 8 Derating Curve

AP8800A

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Application Information (cont.) (@T

Application Example

The typical application for the AP8800A is the MR16 application driving 1W rated LEDs from a 12V ac supply.

= +25°C, VIN = 12V, unless otherwise specified.)

A

VIN

SET

SW

GND

CTRL

P8800A

AP8800A Component List

Quantity PCB Ident Value Description Suggested Source

1 U1 AP8800A LED Driver IC Diodes Inc.
1

1 R1 0R15

1 C1 330µF 20V Electrolytic capacitor Kemet

1 C2 1µF > = 25V

1 C3 100nF > = 25V

1 L1 100µH LPS6235 - series Coilcraft

D1, D2, D3,

D4, D5

SBR1A40 Input bridge & freewheeling diode Diodes Inc.

Resistor, 0805, +/-1% <+/-300ppm
KOA SR732ATTDR150F

X7R 1206 Generic
Kemet C1206105K5RAC7800 (50v)
NIC NMC1206X7R105K50F (50v)

X7R 0805 Generic Kemet
C0805C104K5RAC (50v)
NIC NMC0805X7R104K50TRPF (50v)

Kemet

Kemet NIC
Components

Kemet NIC
Components

AP8800A

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θ

Ordering Information

P8800A

Part Number Package Code Packaging

AP8800AWT-7 WT TSOT25 3000/Tape & Reel -7

Quantity Part Number Suffix

7” Tape and Reel

Marking Information

(1) TSOT25

(Top View)

W

7

4

XX

: Identification code

Y

: Year 0~9

X

W

: Week : A~Z : 1~26 week;

a~z : 27~52 week; z represents
52 and 53 week

X

: A~Z : Internal code

5

Y

XX

1 2 3

Part Number Package Identification Code

AP8800AWT-7 TSOT25 AZ

Package Outline Dimensions (All dimensions in mm.)

Please see AP02002 at http://www.diodes.com/datasheets/ap02002.pdf for latest version.

E1

A2

A

A1

D

e1

e

AP8800A

Document number: DS35100 Rev. 3 - 2

5x b

E

L2

c

L

4x 1

θ

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TSOT25

Dim Min Max Typ

A

−

A1 0.01 0.10
A2 0.84 0.90

D

− −

E

− −

E1

− −

b 0.30 0.45
c 0.12 0.20
e

− −

e1

− −

L 0.30 0.50

L2

− −

θ 0° 8° 4°

θ1 4° 12°

All Dimensions in mm

1.00

−

−

−

2.90

2.80

1.60

−

−

0.95

1.90

0.25

−

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P8800A

Suggested Pad Layout

Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for the latest version.

DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
(AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION).

Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes
without further notice to this document and any product described herein. Diodes Incorporated does not assume any liability arising out of the
application or use of this document or any product described herein; neither does Diodes Incorporated convey any license under its patent or
trademark rights, nor the rights of others. Any Customer or user of this document or products described herein in such applications shall assume
all risks of such use and will agree to hold Diodes Incorporated and all the companies whose products are represented on Diodes Incorporated
website, harmless against all damages.

Diodes Incorporated does not warrant or accept any liability whatsoever in respect of any products purchased through unauthorized sales channel.
Should Customers purchase or use Diodes Incorporated products for any unintended or unauthorize d application, Customers shall indemnify and
hold Diodes Incorporated and its representatives harmless against all claims, damages, expenses, and attorney fees arising out of, directly or
indirectly, any claim of personal injury or death associated with such unintended or unauthorized application.

Products described herein may be covered by one or more United States, international or foreign patents pending. Product names and markings
noted herein may also be covered by one or more United States, international or foreign trademarks.

Diodes Incorporated products are specifically not authorized for use as critical components in life support devices or systems without the express
written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:

A. Life support devices or systems are devices or systems which:

1. are intended to implant into the body, or

2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the

labeling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the

failure of the life support device or to affect its safety or effectiveness.
Customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support devices or systems, and

acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any
use of Diodes Incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systems-related
information or support that may be provided by Diodes Incorporated. Further, Customers must fully indemnify Diodes Incorporated and its
representatives against any damages arising out of the use of Diodes Incorporated products in such safety-critical, life support devices or systems.

Copyright © 2012, Diodes Incorporated

www.diodes.com

Y1

C C

X (5x)

Y (5x)

IMPORTANT NOTICE

LIFE SUPPORT

Dimensions Value (in mm)

C

X
Y

Y1

0.950

0.700

1.000

3.199

AP8800A

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