NCP3020A: Buck-Boost Converter for 3 A LEDs

Design Note – DN05002/D
Buck-Boost Converter for 3 A LEDs
Input Voltage
Output Power
I/O Isolation
high current LED
10 to 25 Vdc
60 Watts
Output Voltage
Nominal Current
Nominal Efficiency
15 – 22Vdc
1 – 3A
82 - 89%
There are situations when a converter is required to perform as a boost or a buck and to seamlessly shift
from one mode to the other depending on the output to input voltage ratio. For example if 18 V / 2 A are
required to drive a string of power LEDs while the input power comes from a 12 or 18V battery pack, this
translates into a 10 to 25 V input range for the converter.
Since LEDs don’t necessarily need a drive voltage that’s referenced to ground, a “floating” boost
arrangement can be used and this pseudo buck-boost can be both simple and efficient. During the ON
time, the input current flows from the input supply through the inductor and MOSFET to GND. During the
OFF time, the inductor keeps flowing and its path is closed through the load.
This Design Note describes such a circuit which operates from 10 to 25 V input and produces 15 to 22 V
in order to drive a string of several power LEDs at up to 3 A.
May 2011, rev. 0
Circuit Description
Having a 5 to 28 V input range, the NCP3020A buck controller is well suited for the application. The high
side driver is used to drive the boost MOSFET while the low side one is left open.
The output current is sensed by Rcs1 resistor and the current mirror circuit consisting of Q2 – Q5 makes
sure a fraction of this current flows through resistor R3. The voltage across R3 is then fed to the FB pin
of the controller.
Q6 and adjacent components help implement a soft start function. At power-up, Q6 is initially off and
resistor R9 appears in series with R3, ensuring a low current at start. Then the voltage across C10
slowly increases and Q6 gradually turns on and shunts R9 and from that moment on, R3 alone controls
the output current.
The ON-OFF switch is placed in a low current path providing power just for the controller. This
arrangement allows a low cost switch to be used, but the user must be aware that approximately 0.5 mA
of leakage current exists due to the current mirror. If this situation is not acceptable, than the shown
switch should be by-passed and a series power MOSFET should be used in the power path to turn on
and off the converter.
MOSFET Q1 and diode D1 may seem oversized for a circuit required to provide up to 3 A, but peak
currents can reach in excess of 10 A and efficiency will suffer if weaker parts are used. The efficiency
was measured with an electronic load simulating a string of LED’s requiring 22 V at up to 3 A.
May 2011, rev.0
May 2011, rev.0
Transient Response
For this test, a second load was switched on and off in parallel with the main load. The converter
successfully maintains a constant output current. The transient spikes are totally acceptable since
this application does not require a fast response.
Output Current Adjustment
May 2011, rev.0
Output current is set by resistor R2 which can be chosen according to the table below.
Iout (A)
R2 (Ω)
Key Features
Seamless “buck to boost” transition
High efficiency
Constant output current
 Data sheet NTMFS5844NL: 60 V, 60 A, 12 m Ohm Single N-Channel Power MOSFET in SO-8FL
 Data sheet NCP3020: 28V Synchronous Buck Controller
© 2011 ON Semiconductor.
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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 Michael Borza, e-mail: [email protected]
May 2011, rev.0