NCP3063: 9 V to 36 Vin dc, Constant Current LED Driver

DN06001/D
Design Note – DN06001/D
9 V to 36 Vin dc, Constant Current LED Driver
Device
Application
NCP3063
DC-DC Constant
Current LED Driver
Input Voltage
Output Power
Topology
I/O Isolation
10 W
Non Inverting
Buck Boost
None
9 Vdc to 36 Vdc
Other Specifications
Output Voltage
Ripple
Max Current
Output 1
Output 2
Output 3
Output 4
12 Vdc
200 mV
1A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PFC (Yes/No)
No
Minimum Efficiency
Cooling Method/Supply
Orientation
70%
Convection
Circuit Description
Key Features
ON
Semiconductor’s
latest
monolithic
NCP3063 control circuit and family of
e2PowerEdge bipolar transistors are combined
to create a non inverting buck boost topology
optimized to drive strings of LED’s at a constant
current.
A high side, low drop, current sensing scheme
has been implemented, targeted for automotive
and other high efficiency applications.
The output from the current sense is used to
vary the slope of the oscillator ramp and
achieve duty cycle modulation, independent of
the gated oscillator function provided on the IC.
• Seamless buck or boost operation
• Constant current output with voltage clamp
• Low drop high side current sensing
• Baker clamp drive on external transistor for
enhanced efficiency
• High frequency operation to enable cost
effective magnetic and capacitive (e.g. MLCC)
filter components
• Minimal ripple current through LED string
September 2006, Rev. 1
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1
DN06001/D
Schematic
Design Notes
For low ripple current assume CCM operation. Switches Q1 and Q2 turn on for time D*Ts (D duty cycle, Ts switching
period) charging inductor L1 from input Vin. When Q1 and Q2 turn off, diodes D1 and D2 deliver inductor energy to output
Vout.
For the inductor flux (volt microsecond) to remain in equilibrium after each switching cycle, Vin*D*Ts must equal
Vout*(1-D)*Ts, neglecting circuit losses. Hence voltage gain of buck boost is given by Vout = Vin*D/(1-D). When D=0.5,
Vout = Vin.
Ripple current in inductor obtained from expression ΔI(L1) = Vin*D*Ts/L1.
A value for L1 of 150 µH will maintain a +/-15% ripple current in a 700 mA application (3 watt LED’s) assuming Vin =12 V
and 200 kHz operation (D*Ts = 0.5*5 µs).
The base current for Q1 and Q2 is determined by Vin/R3. (100:1 base drive good starting point but remember to include
Vbe/R2 contribution). Q1 and Q2 are turned off passively via base resistors R2 and R4. To improve switch off times, switch
Q2 may be held out of saturation by the Baker clamp created by diodes D3a and D3b. Several points of efficiency
improvement may be achieved using this technique. Low drop current sensing is achieved across current sense resistor
R8.
The constant output current is determined from the equation Io*R8 = (Vbe(Q3a)/R6)*R7 where Vo/R9 = 2*Vbe(Q3a)/R6.
Vclamp equals Vz(D4) + 1.25 V. Alternatively, if D4 is replaced with a resistor R11 then Vclamp = (1+R11/R1)*1.25 volts.
September 2006, Rev. 1
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DN06001/D
Converter Waveforms
Boost Mode from 8 Vin to 16 Vout
Buck Mode from 16 Vin to 8 Vout
September 2006, Rev. 1
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DN06001/D
Efficiency data of Buck-Boost converter between 8 Vin dc and 26 Vin dc driving
LED strings at 350 mA and 700 mA 1
OSTAR4 driven at 350mA
90
85
Buck/Boost Efficiency
80
Vo([email protected])
Vo([email protected])
Vo([email protected])
75
70
65
60
5
10
15
20
25
30
Input Voltage Range
OSTAR4 driven at 700mA
90
85
Buck/Boost Efficiency
80
Vo([email protected])
Vo([email protected])
Vo([email protected])
75
70
65
60
5
10
15
20
25
30
Input Voltage Range
1
© 2006 ON Semiconductor.
Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor
does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to
assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated
with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its
products at any time, without notice.
Design note created by Dennis Solley, e-mail: [email protected]
September 2006, Rev. 1
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4