AP8800A

AP8800A
COST EFFECTIVE 28V BUCK 1-W LED DRIVER
Description
Pin Assignments
The AP8800A is a step-down DC/DC converter designed to drive
(Top View)
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,
SW
1
GND
2
CTRL
3
5
VIN
4
SET
hence minimizing the PCB area needed.
Maximum output current of AP8800A is set via an external resistor
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 switches off the output
MOSFET simplifying PWM dimming.
Features
Applications
•
LED driving current up to 370mA
•
MR16 Lamps
•
Better than 5% accuracy
•
General Illumination Lamps
•
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
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
AP8800A
Document number: DS35100 Rev. 3 - 2
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AP8800A
Pin Descriptions
Pin Name
SW
GND
Pin Number
1
2
CTRL
3
SET
4
VIN
5
Function
Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to reduce EMI.
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.
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 VCTRL = VREF.
• Drive with DC voltage (0.3V < VCTRL < 2.5V) to adjust output current from 24% to 200% of IOUTnom
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
Absolute Maximum Ratings (@TA = +25°C, unless otherwise specified.)
Symbol
VIN
VSET
VSW
VCTRL
ISW
TJ
TLEAD
TST
Caution:
Parameter
Continuous VIN Pin Voltage Relative to GND
Set Voltage Relative to VIN
SW Voltage Relative to GND
CTRL Pin Input Voltage
Switch Current
Junction Temperature
Lead Temperature Soldering
Storage Temperature Range
Ratings
-0.3 to +30
VIN -5 to VIN +0.3
-0.3 to +30
-0.3 to +5
0.45
105
300
-55 to +150
Unit
V
V
V
V
A
°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.
AP8800A
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AP8800A
Recommended Operating Conditions (@TA = +25°C, unless otherwise specified.)
Symbol
Min
Max
Unit
Operating Input Voltage Relative to GND
8.0
28
V
VCTRLDC
Voltage Range for 24% to 200% DC Dimming Relative to GND
0.3
2.5
V
VCTRLL
Voltage Low for PWM Dimming Relative to GND
0.2
V
370
mA
VIN
ISW
DPWM
Parameter
Continuous Average Switch Current (Note 4)
Duty Cycle Range Applied to CTRL (f < 500Hz)
fSW
Switching Frequency
TA
Ambient Temperature Range
Note:
0.01
1
600
kHz
-40
105
°C
4. Dependent on junction and ambient temperature.
Electrical Characteristics (@TA = +25°C, VIN = 12V, unless otherwise specified.)
Symbol
Typ
Max
Unit
IQ
Quiescent Current
Parameter
VCTRL = 1.25V, fSW = 250kHz
300
650
µA
ISD
Shutdown Current
VCTRL = 0V
20
30
µA
100
105
mV
VTHD
Internal Threshold Voltage
VREF
Internal Reference Voltage
ISET
SET Pin Input Current
RDS(ON)
On Resistance of MOSFET
ISW-LKG
Switch Leakage Current
fSW
Switching Frequency with L = 100µH
tSS
Soft-Start Time
tPD
Internal Comparator Propagation Delay
θJA
Note:
Thermal Resistance Junction-to-Ambient
Conditions
Min
95
1.25
VSET = VIN-0.1
1.3
1.5
ADJ pin floating L = 100H
IOUT = 350mA @ 1 LED (VLED = 3.4V)
Time taken for output current to reach 90%
of final value after VCTRL = 0.3V
TSOT25 (Note 5)
V
µA
2.2
Ω
5
µA
250
kHz
500
µs
55
ns
200
°C/W
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.
AP8800A
Document number: DS35100 Rev. 3 - 2
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AP8800A
Typical Characteristics
25
500
TA = 25°C
TA = 25°C
20
Shutdown Current (µA)
Quiescent Current (µA)
400
300
200
15
10
100
0
0
5
0
4
0
12
16
20
24
28
Input Voltage (V)
Shutdow n Current (Non-Sw itching) vs. Input Voltage
1.255
8
12
16
20
24
28
Input Voltage (V)
Quiescent Current (Switching) vs. Input Voltage
1.2425
4
TA = 25°C
8
V IN = 12V
V REF (V)
Reference Voltage (V)
1.25
1.242
1.245
1.24
1.2415
4
8
12
16
20
24
Input Voltage (V)
Reference Voltage vs. Input Voltage
28
1.235
-40
1.4
V IN = 12V
Power Switch On-Resistance ( Ω)
1.2
Reference Voltage (V)
10
35
60
85
110
Ambient Temperature ( °C)
Reference Voltage vs. Ambient Temperature
2.4
TA = 25°C
1
0.8
0.6
0.4
0.2
0
-15
2.2
2
1.8
1.6
1.4
1.2
0
1
2
1
-40
3
4
5
6
7
8
9 10
Input Voltage (V)
Reference Voltage vs. Input Voltage (Low Voltage)
AP8800A
Document number: DS35100 Rev. 3 - 2
110
10
35
60
85
Ambient Temperature (°C)
Power Switch On-Resistance vs. Ambient Temperature
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AP8800A
Typical Characteristics
(cont.)
350
RS = 0.3 Ω
V IN = 12V
L = 100µH
R S = 0.33 Ω
300
R S = 0.56 Ω
1
250
IOUT Mean (mA)
Deviation From Nominal Set Current(%)
2
0
200
RS = 1 Ω
150
100
-1
50
-2
-40
0
-15
10
35
60
85
110
Ambient Temperature (°C)
Change in LED Current vs. Ambient Temperature
AP8800A
Document number: DS35100 Rev. 3 - 2
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0.5
1.5
2
V CTRL (V)
Output Current vs. VCT RL
1
2.5
3
August 2012
© Diodes Incorporated
AP8800A
Application Information (@TA = +25°C, VIN = 12V, unless otherwise specified.)
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 +VIN (See Figure 1), the AP8800A 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).
Figure 2 Typical Switching Waveform
Figure 1 Typical Configuration
This rising current produces a voltage ramp across RSET. The internal circuit of the AP8800A 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 AP8800A. 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
With the CTRL pin left floating and the external current sense resistor, RSET (greater than 0.3 Ω) is connected between VIN and SET, the nominal
average output current in the LEDs is:
ILED =
VTH
R SET
where VTH is nominally 100mV
AP8800A
Document number: DS35100 Rev. 3 - 2
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AP8800A
Application Information (cont.) (@TA = +25°C, VIN = 12V, unless otherwise specified.)
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.
Switching Frequency
700
600
100
47
Frequency [kHz]
500
150
400
12V - 1LED
68
24V - 3 LEDs
220
300
28V - 4LEDs
100
200
150
470
220
100
470
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
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 the reverse
leakage of the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device.
AP8800A
Document number: DS35100 Rev. 3 - 2
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AP8800A
Application Information (cont.) (@TA = +25°C, VIN = 12V, unless otherwise specified.)
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
V
ILED = CTRL × TH
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.
LED current Versus RSET and VCTRL
400
0.56
350
0.3
LED Current [mA]
300
0.33
0.39
250
LED current @
VCTRL = 1.25V
0.68
0.75
0.82
0.47
200
0.56
0.68
0.3
150
0.39
0.47
0.56
100
50
LED current @
VCTRL = 2.5V
1
1.2
0.82
0.68
1
1.2
0.82
LED current @
VCTRL = 0.625V
1.5
1
1.2
2
1.5
1.5
2
3
2
3
3
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
RSET value [Ohms]
Figure 4 LED Current Setting vs. RSET and VCTRL
AP8800A
Document number: DS35100 Rev. 3 - 2
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AP8800A
Application Information (cont.) (@TA = +25°C, VIN = 12V, unless otherwise specified.)
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 RSET. PWM dimming gives a wider average LED current variation and is more accurate at lower average LED currents than by
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.
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
Document number: DS35100 Rev. 3 - 2
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AP8800A
Application Information (cont.) (@TA = +25°C, VIN = 12V, unless otherwise specified.)
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 switching frequency and duty cycle will change. If one or more 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.
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
Power dissipation (mW)
350
300
250
200
150
100
50
0
-40
-25
-10
5
20
35
50
65
80
95
110
Ambient temperature (°C)
Figure 8 Derating Curve
AP8800A
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AP8800A
Application Information (cont.) (@TA = +25°C, VIN = 12V, unless otherwise specified.)
Application Example
SET
CTRL
GND
VIN
SW
The typical application for the AP8800A is the MR16 application driving 1W rated LEDs from a 12V ac supply.
AP8800A Component List
Quantity
PCB Ident
Value
1
U1
AP8800A
LED Driver IC
Diodes Inc.
1
D1, D2, D3,
D4, D5
SBR1A40
Input bridge & freewheeling diode
Diodes Inc.
1
R1
0R15
Resistor, 0805, +/-1% <+/-300ppm
KOA SR732ATTDR150F
Kemet
1
C1
330µF 20V
Electrolytic capacitor
Kemet
1
C2
1µF > = 25V
1
C3
100nF > = 25V
1
L1
100µH
AP8800A
Document number: DS35100 Rev. 3 - 2
Description
X7R 1206 Generic
Kemet C1206105K5RAC7800 (50v)
NIC NMC1206X7R105K50F (50v)
X7R 0805 Generic
C0805C104K5RAC (50v)
NIC NMC0805X7R104K50TRPF (50v)
LPS6235 - series
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Suggested Source
Kemet
Kemet
Components
NIC
Kemet
Components
NIC
Coilcraft
August 2012
© Diodes Incorporated
AP8800A
Ordering Information
Part Number
Package Code
Packaging
AP8800AWT-7
WT
TSOT25
Quantity
3000/Tape & Reel
7” Tape and Reel
Part Number Suffix
-7
Marking Information
(1) TSOT25
(Top View)
4
7
5
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
X : A~Z : Internal code
XX Y W X
1
2
3
Part Number
AP8800AWT-7
Package
TSOT25
Identification Code
AZ
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
AP8800A
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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
© Diodes Incorporated
AP8800A
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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written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
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labeling can be reasonably expected to result in significant injury to the user.
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failure of the life support device or to affect its safety or effectiveness.
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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
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AP8800A
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