Diodes AP8800 User Manual

A

P8800

350mA LED STEP-DOWN CONVERTER

Description

The AP8800 is a step-down DC/DC converter designed to drive LEDs
with a constant current. The device can drive up to seven 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 eliminates
the need for ballast resistors. The AP8800 switches at frequency up
to 0.6MHz. This allows the use of small size external components,
hence minimizing the PCB area needed.

Maximum output current of AP8800 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 shuts down the output
at SW and puts the device into a low-current standby state.

and SET input pins. Dimming is achieved

IN

Features

• LED driving current up to 350mA

• Compatible with 12V & 24V standard systems

• High efficiency up to 92%

• High switching frequency up to 0.6MHz

• PWM/DC input for dimming control

• Built-in soft-start function

• Built-in output open-circuit protection

• SO-8, MSOP-8 and U-DFN3030-10: 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)

Applications

• Commercial & Industrial Lighting

• Small LCD Panel Backlight

• Architecture Detail Lighting

• Appliances Interior Lighting

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.

Pin Assignments

CTRL

RSVD

SW

GND

CTRL

RSVD

GND

CTRL

RSVD

SW

GND

SO-8

( Top View )

1

2
3

AP8800

8
7
6

45

MSOP-8

( Top View )

SW

1
2

3

AP8800

8
7
6

45

U-DFN3030-10

( Top View )

NC

1

2

3

4

5

AP8800

10

9

8

7

6

SET

NC

NC

V

SET

NC

NC

V

IN

SET
NC

NC

NC
V

IN

IN

Typical Applications Circuit

AP8800

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Pin Descriptions

Pin Name

SO-8 MSOP-8 U-DFN3030-10

SW 3 3 4

Pin Number

Function

Switch Pin. Connect inductor/freewheeling diode here. Minimize trace area at this pin to
reduce EMI.

GND 4 4 5 GND Pin

SET 8 8 10 Set Nominal Output Current Pin. To configure the output current of the device.

Dual function dimming control pin.

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

CTRL 1 1 1

• A PWM signal (driven by an open-drain/collector source) allows the output current
to be adjusted over a wide range up to 100%.

• An analog voltage between 0.3V and 2.5V adjusts the output current between 25%
and 200% of the current set by 0.2V/R

The input impedance is about 200kΩ, and if the pin is left open V

.

S

CTRL

VIN 5 5 6 Input Supply Pin. Must be locally bypassed.

RSVD 2 2 2 Reserved. Normally connected to Ground

NC 6, 7 6, 7 2, 7, 8, 9 No Connection.

Functional Block Diagram

= V

REF

P8800

Absolute Maximum Ratings (@T

= +25°C, unless otherwise specified.)

A

Symbol Parameter Rating Unit

V

VIN Pin Voltage -0.3 to +30 V

IN

V

Set Voltage Relative to VIN V

SET

-5 to VIN +0.3 V

IN

VSW SW Voltage -0.3 to +30 V
V

CTRL Pin Input Voltage -0.3 to +5 V

CTRL

T

Maximum Junction Temperature 125 °C

J

T

Maximum Lead Temperature 300 °C

LEAD

T

Storage Temperature Range -55 to +125 °C

ST

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

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.

AP8800

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P8800

Recommended Operating Conditions (@T

= +25°C, unless otherwise specified.)

A

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 (Note 2) 0.3 2.5 V

CTRLDC

V

Voltage Low for PWM Dimming Relative to GND 0 0.2 V

CTRLL

f

Maximum Switching Frequency 500 kHz

OSC

TA Ambient Temperature Range -40 +85 °C

Duty Cycle Using Inductor ≥ 100µH (Note 3) 0.1 0.95

V

CTRL Input Voltage to Attain 100% LED Current 1.25

ENH

V

CTRL Input Voltage Below Which Device Turns Off 0.2

ENL

Notes: 2. For 100% brightness either leave floating or connect to 1.25V relative to GND.

3. For most applications the LED current will be within 8% over the duty cycle range specified. Duty cycle accuracy is also dependent on propagation
delay. Smaller size inductors can be used but LED current accuracy may be greater than 8% at extremes of duty cycle. This is most noticeable at low
duty cycles (less than 0.1) or when the input voltage is high and only one LED is being driven.

V

Electrical Characteristics (@T

= +25°C, unless otherwise specified.)

A

Symbol Parameter Conditions Min Typ Max Unit

I

Continuous Switch Current (Note 4) — — 350 mA

OUT

IQ Quiescient Current — 20 30 μA

V

Internal Current Sense Threshold Voltage V

THD

V

Internal Reference Voltage — 1.25 — V

REF

SET SET Pin Input Current V

– V

92 100 108 mV

IN

SET

= V

-0.1 — 1.3 — μA

SET

IN

RDS(on) On-Resistance of Internal Switch — 1.7 2.2 Ω

I

SW_Leakage

Notes: 4. Refer to figure 8 for the device derating curve.

5. Test condition for SO-8, MSOP-8 and U-DFN3030-10: Device mounted on FR-4 PCB, 2”x 2”, 2oz 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.

Switch Pin Leakage Current — — 5 μA

SO-8 (Note 5) — 92 —

θ

Thermal Resistance Junction-to-Ambient

JA

MSOP-8 (Note 5) —
U-DFN3030-10 (Note 5) —
SO-8 (Note 5) —

θ

Thermal Resistance Junction-to-Case

JC

MSOP-8 (Note 5) —
U-DFN3030-10 (Note 5) —

120

46
60
98
32

—
—
—
—
—

°C/W
°C/W
°C/W
°C/W
°C/W
°C/W

AP8800

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P8800

Application Information

AP8800 Operation

The AP8800 is a hysteretic LED current switching regulator sometimes known as an equal ripple switching regulator. In normal ope ration, 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 AP8800 internal switch is turned on. Current starts to flow through sense resistor R1, inductor L1,

IN

, 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

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

R

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.

AP8800

Document number: DS31764 Rev. 7 - 2

. The internal circuit of the AP8800 senses the voltage across R

SET

, which is sensed by the AP8800. A voltage proportional to the sense voltage across

SET

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Figure 2 Typical Switching Waveform

SET

with a switching current

SET,

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and applies a

SET

August 2012

,