AP8803

AP8803
30V 1A BUCK LED DRIVER
Description
Pin Assignments
The AP8803 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
SW 1
voltage source of 8V to 30V. Series connection of the LEDs provides
5 V IN
identical LED currents resulting in uniform brightness and eliminating
the need for ballast resistors. The AP8803 switches at frequency up
GND 2
to 700kHz. This allows the use of small size external components,
hence minimizing the PCB area needed.
CTRL 3
4 SET
Maximum output current of AP8803 is set via an external resistor
Top View
connected between the VIN and SET input pins. Dimming is achieved
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.
Features
•
LED Driving Current up to 1A
•
High Efficiency Up to 92%
•
Operating Input Voltage Up to 30V
•
High Switching Frequency Up to 700kHz
•
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
ƒ
Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2)
ƒ
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.
Typical Applications Circuit
12V_DC
VIN
RSET
CTRL
SET
AP8803
L1
C1
GND
AP8803
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AP8803
Pin Descriptions
Pin Name
SW
GND
SET
CTRL
VIN
Function
Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to reduce EMI.
GND pin
Set Nominal Output Current Pin. Configure the output current of the device.
Dimming and On/Off Control Input.
•
Leave floating for normal operation.
(VCTRL = VREF = 1.25V giving nominal average output current IOUTnom = 0.1/RS)
•
Drive to voltage below 0.2V to turn off output current
•
Drive with DC voltage (0.3V < VCTRL < 1.25V) to adjust output current from 25% to 100% of IOUTnom
•
Input voltage of 0.2V or lower forces the device into low current standby mode and shuts off the output. A PWM signal
allows the output current to be adjusted above or below the level set by the resistor connected to SET input pin.
Input Supply Pin. Must be locally decoupled to GND with > 2.2µF X7R ceramic capacitor.
Functional Block Diagram
Absolute Maximum Ratings (@TA = +25°C, unless otherwise specified.)
Symbol
VIN
VSW
VCTRL
ISW
ESDHBM
ESDMM
TJ
TLEAD
TST
Caution:
Parameter
Continuous VIN pin voltage
Transient (t < 0.5s)
SW voltage
Transient (t < 0.5s)
CTRL pin input voltage
Switch current
Human Body Model ESD
Machine Model ESD
Junction Temperature
Lead Temperature Soldering
Storage Temperature Range
Rating
-0.3 to +30
40
-0.3 to +30
40
-0.3 to +6
1.25
500
<100
150
300
-65 to +150
Unit
V
V
V
A
V
V
°C
°C
°C
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
AP8803
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AP8803
Recommended Operating Conditions (@TA = +25°C, unless otherwise specified.)
Symbol
VIN
VCTRLH
VCTRLL
ISW
TA
Duty Cycle
Notes:
Parameter
Min
Max
Unit
8.0
0.3
V
V
V
A
°C
Operating Input Voltage
Voltage High
Voltage Low
Continuous Switch Current (Note 5)
Ambient Temperature Range
-40
30
2.5
0.25
1
+125
Using Inductor ≥ 100µH (Note 4)
0.1
0.95
4. 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.
5. Refer to Figure 8 for the device derating curve.
Electrical Characteristics (@TA = +25°C, unless otherwise specified.)
Symbol
Parameter
VINSU
VINSD
IQ
VTHD
ISET
VREF
RDS(on)
ISW
Internal Regulator Start-Up Threshold
Internal Regulator Shutdown Threshold
Quiescent Current
Internal Threshold Voltage
SET Pin Input Current
Internal Reference Voltage
On Resistance of MOSFET
Continuous Switch Current
Switch Leakage Current
Switching Frequency
Thermal Resistance Junction-toAmbient
ISW_Leakage
fOSC
θJA
Notes:
Conditions
Min
Typ
92
5.65
5.55
1.8
100
1.25
1.25
0.5
VIN rising
VIN falling
CTRL pin floating f = 250kHz
VSET = VIN -0.1
ISW = 1A
(Note 5)
TSOT25 (Note 6)
Max
Unit
V
V
mA
mV
µA
V
Ω
A
µA
MHz
5
108
10
1
1
8
0.7
125
°C/W
5. Refer to Figure 8 for the device derating curve.
6. Test condition for TSOT25: Device mounted on FR-4 PCB, 25mm x 25mm, 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.
AP8803
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AP8803
Application Information
AP8803 Operation
The AP8803 is a hysteretic LED current switching regulator sometimes known as an equal ripple switching regulator. In normal operation, when
voltage is applied at +VIN (See Figure 1), the AP8803 internal switch is turned on. Current starts to flow through sense resistor R1, inductor L1,
and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +VIN, and the inductor L1 (See Figure 2).
12V_DC
VIN
RSET
CTRL
SET
AP8803
L1
C1
GND
SW
Figure 2 Typical switching waveform
Figure 1 Typical configuration
This rising current produces a voltage ramp across RSET. The internal circuit of the AP8803 senses the voltage across RSET and applies a
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
RSET, L1, the LEDs and the schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined by the forward
voltage drop of the LEDs and the schottky diode.
This decaying current produces a falling voltage at RSET, which is sensed by the AP8803. A voltage proportional to the sense voltage across
RSET 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 again. This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor RSET, with a switching current
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.
LED Current Control
The LED current is controlled by the resistor RSET in Figure 1.
Connected between VIN and SET the nominal average output current in the LED(s) is defined as:
ILED =
VTH
R SET
where VTH is nominally 100mV
AP8803
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AP8803
Application Information (cont.)
Inductor Selection
A 33μH inductor (or higher) is recommended for most AP8803 applications with input voltage at 24V.
Figure 3 displays the resulting switching frequency varying the main circuit parameters: Supply voltage, inductor value and number of LEDs to
be driven.
In particular, the graph in Figure 3 gives values of nominal switching frequency for several values of inductors (L1) in the typical application
circuit shown on Figure 1, for different input voltages and load condition. It can be used to determine the inductor value based on the desired
switching frequency and the input and load conditions.
Switching Frequency @ ILED=1A
600
Frequency [kHz]
500
33
400
47
12V - 1 LED
68
300
30V - 5 LEDs
100
33
200
24V - 3 LEDs
150
47
68
100
100
220
470
150
220
0
0
50
100
150
200
250
300
350
400
450
500
Inductor Value [uH]
Figure 3 Switching Frequency vs. Supply Voltage, Inductor, and Number of LEDs
Capacitor Selection
The small size of ceramic capacitors makes them ideal for AP8803 applications. X5R and X7R types are recommended because they retain
their capacitance over wider voltage and temperature ranges than other types such as Z5U. A 2.2μF input capacitor is sufficient for most
intended applications of AP8803.
Diode Selection
Schottky diodes, e.g. B240 or DFLS240L in the proprietary PowerDI123 package, with their low forward voltage drop and fast reverse recovery,
are the ideal choice for AP8803 applications.
In addition, Super Barrier Rectifier devices (as SBR2A40P1) can be used for their enhanced thermal performances.
AP8803
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AP8803
Application Information (cont.)
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 is:
ILED =
VCTRL VTHD
VREF R SET
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 (RSET) in the typical application
circuit shown on Figure 1, for different voltages applied on the CTRL pin.
It can be used to determine the RSET value based on the desired LED current and the condition of the CTRL pin (floating or driven with an
external DC voltage lower than 2.5V and higher than 0.2V).
LED current Versus RSET and VCTRL
1200
0.2
LED Current [mA]
1000
LED current @
VCTRL = 1.25V
800
LED current @
VCTRL = 2.5V
0.3
600
0.1
0.39
400
0.15
LED current @
VCTRL = 0.625V
0.47
0.75
0.3
200
0.82
1
1.5
0.47
0.75 0.82
0
0
1
1.5
2
3
3
2
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
RSET value [Ohms]
Figure 4 LED Current Setting vs. RSET and VCTRL
AP8803
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AP8803
Application Information (cont.)
PWM Dimming
A Pulse Width Modulated (PWM) signal with a max resolution of 8-bit, can be applied to the CTRL pin to change the output current to a value
above or below the nominal average value set by resistor RSET. To achieve this resolution the PWM frequency has to be lower than 500Hz.
Figure 5 shows the typical PWM response of the AP8800A. An internal filter produces a rump
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 internal 200kΩ resistor between the CTRL 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.
AP8803
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AP8803
Application Information (cont.)
Fault Condition Operation
The AP8803 has by default open LED protection. If the LEDs should become open circuit the AP8803 will stop oscillating; the SET pin will rise
to VIN and the SW pin will then fall to GND. No excessive voltages will be seen by the AP8803.
If the LEDs should become shorted together the AP8803 will continue to switch however the duty cycle at which it will operate will change
dramatically and the switching frequency will most likely decrease. The on-time of the internal power MOSFET switch will be significantly
reduced because almost all of the input voltage is now developed across the inductor. The off-time will be significantly increased because the
reverse voltage across the inductor is now just the Schottky diode voltage (See Figure 7) causing a much slower decay in inductor current.
During this condition the inductor current will remain within its controlled levels and so no excessive heat will be generated within the AP8803.
Figure 7 Switching Characteristics (normal open to short LED chain)
Thermal Considerations
The graph below in Figure 8, gives details for power dissipation derating. This assumes the device to be mounted on a 25 x 25mm PCB with
1oz copper standing in still air.
Power dissipation (W)
1.2
1
0.8
0.6
0.4
0.2
0
-50
-25
0
25
50
75
100
125
150
Ambient Temperature (°C)
Figure 8 Power Dissipation Derating Curve
The maximum power dissipation is affected by PCB area and the area of copper associated with the LX pin as well as other components on the
PCB generating heat – such as the inductor, capacitor or rectifiers.
AP8803
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AP8803
Ordering Information
Part Number
Package Code
Packaging
AP8803WTG-7
WT
TSOT25
Quantity
3000/Tape & Reel
7” Tape and Reel
Part Number Suffix
-7
Marking Information
XX : Identification Code
Y : Year 0~9
W : Week: A~Z: 1~26 week
a~z: 27~52 week;
z represents 52 and 53 week
Part Number
Package
Identification Code
AL8803WT-7
TSOT25
AY
Package Outline Dimensions (All dimensions in mm.)
Please see AP02002 at http://www.diodes.com/datasheets/ap02002.pdf for latest version.
D
e1
E
E1
L2
c
4x θ1
e
L
θ
5x b
A
A2
A1
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TSOT25
Dim Min Max Typ
A
1.00
−
−
A1
0.01 0.10
−
A2
0.84 0.90
−
D
2.90
−
−
E
2.80
−
−
E1
1.60
−
−
b
0.30 0.45
−
c
0.12 0.20
−
e
0.95
−
−
e1
1.90
−
−
L
0.30 0.50
L2
0.25
−
−
θ
0°
8°
4°
θ1
4°
12°
−
All Dimensions in mm
August 2012
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AP8803
Suggested Pad Layout
Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for the latest version.
C
C
Dimensions Value (in mm)
C
0.950
X
0.700
Y
1.000
Y1
3.199
Y1
Y (5x)
X (5x)
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