LED Driver. AN874 Datasheet

AN874 Driver. Datasheet pdf. Equivalent


Microchip AN874
AN874
Buck Configuration High-Power LED Driver
Author: Keith Curtis
Microchip Technology Inc.
INTRODUCTION
The circuit and firmware described in this application
note demonstrates a minimal parts count driver/control-
ler for a high-power (1W or greater) LED. The circuit is
based on a buck topology switching power supply
using the on-chip comparator peripheral within the
PIC12F675 PIC® microcontroller. The switching power
supply design ensures efficient power transfer between
the system battery and the output LED.
The Flash PIC microcontroller is used to implement
intensity control, automated intensity compensation for
low battery conditions, and the ability to playback pre-
programmed Flash sequences. The Flash PIC micro-
controller also allows the creation of a custom PC-
based graphical user interface, using the PICkit™ 1
Flash Starter Kit for programming the pre-programmed
Flash sequences. The combination of the switching
power supply design and the microcontroller results in
an efficient circuit with advanced automated features
while keeping the circuit simple and inexpensive.
High-Power LEDs
The new generation of high-power LED modules repre-
sent a significant advancement in LED design. Ranging
from 1 watt to 10 watts, current high-power LED mod-
ules are capable of delivering between 10 and 50
Lumens/watt of light output. This level of light output is
comparable to most incandescent lamps and even
halogen bulbs in single-color applications. In addition,
high-power LEDs are available in a variety of colors
from multiple manufacturers, in discrete or pre-built
modules.
This application note will focus on driving two different
high-power LED modules, a 1 watt and a 3 watt.
Voltage and Current
The 1 watt chosen module requires a typical drive of
3.4V at 350 mA to produce its full output brightness.
The maximum current drive for the unit is 500 mA.
Exceeding the maximum current specification for the
module, even by short duration pulses, may result in
damage. Therefore, the drive circuitry for the module
must deliver a DC or mostly DC current to the LED
module to produce a full brightness output. The drive
requirements for the 3 watt module are similar, with full
brightness output at 700 mA and a maximum current
drive of 1A.
The circuit described in this application note delivers a
mostly DC drive current with a small triangular ripple
waveform superimposed at the drive level (VDRIVE)
(see Figure 1). The triangular waveform is the result of
the switching nature of the driver, and if kept small in
relation to the DC drive current, will not exceed the
instantaneous maximum current specification for the
LED. In fact, the example designs have been designed
with a safety margin to insure that the instantaneous
current delivered to the LED is always less than the
maximum rating, even when delivering the full-rated
current to the device.
FIGURE 1:
LED Drive
Current
LED CURRENT
VDRIVE
Time
© 2006 Microchip Technology Inc.
DS00874C-page 1


AN874 Datasheet
Recommendation AN874 Datasheet
Part AN874
Description Buck Configuration High-Power LED Driver
Feature AN874; AN874 Buck Configuration High-Power LED Driver Author: Keith Curtis Microchip Technology Inc. INTRO.
Manufacture Microchip
Datasheet
Download AN874 Datasheet




Microchip AN874
AN874
Buck Topology Switching Power Supply
The Buck Topology Switching Power Supply is an
efficient voltage regulator that translates a high-source
voltage into a lower output voltage. It accomplishes this
by rapidly switching the input of an inductor/capacitor
(LC) network between source voltage and ground and
then back to the source voltage (see Figure 2). While
the PWM switch is in position 1, L1 is connected to the
source voltage, the power supply is in its charging
phase, and an increasing current flow (IL) passes from
the source, through the inductor, to the load and COUT.
While the charging current is flowing through the
inductor to the load, part of its energy is stored in the
inductor as a magnetic field. When the PWM switch
changes to position 2, the power supply enters its
discharge phase and the magnetic field around the
inductor collapses, continuing the current flow to the
load. See the graph of the inductor current in Figure 2.
When IL drops to zero, the PWM switch is switched to
position 1 and the charge/discharge cycle starts over.
The result of this switching cycle is an inductor current
that ramps up and down over the course of a cycle.
The capacitor (COUT), in the LC network, acts to
smooth IL into a DC current flow to the load. When IL is
greater than the load current (part B of the capacitor
current graph in Figure 2), the load current is supplied
by IL and any surplus current (IC) flows into COUT,
charging the capacitor. When IL falls below the load
current requirement (part A of the capacitor current
graph), the current flow to COUT reverses and IC
supplements IL to make up the difference between IL
and the required load current.
When describing the operation of the switch control in
a switching power supply design, it is typically the
charge time and total switching time which are
specified. The charge time is specified as a percentage
of the total switching time and is referred to as the duty
cycle of the system, or D in the design equations.
EQUATION 1:
D = -----------------------C----h---a---r---g---e----T---i--m----e------------------------
Charge Time + Discharge Time
The total switching time (represented by T in the
equations) can either be specified directly or by its
reciprocal, the switching frequency (FSWX).
EQUATION 2:
Fswx
=
--------------------------------------1--------------------------------------
Charge Time + Discharge Time
=
-1-
T
FIGURE 2:
BUCK TOPOLOGY SWITCH
CYCLE
Source
Voltage
PWM
1
2
L1
IL
COUT
Ground
+
IC
Load
-
IL Charge Discharge Charge
Inductor
Current
IC
0
Capacitor
Current A
B
AB
A = COUT supplies current to the load.
B = The inductor supplies all of the load current and any
surplus current charges COUT.
A feedback circuit regulates the switching in the switch-
ing power supply. This circuit monitors the load voltage
and compares it to a stable reference. Based on the
result of the comparison, the circuit adjusts the switch-
ing duty cycle to compensate for any discrepancies.
The feedback circuit cancels out any errors in the load
voltage due to component or timing tolerance and it
adjusts the duty cycle to compensate for changes in the
load current. The result is a self-regulating step-down
voltage regulator that produces a stable load voltage
over a variety of load currents.
Note:
A more in-depth explanation of switching
power supplies can be found in the
reference materials listed at the end of this
application note.
DS00874C-page 2
© 2006 Microchip Technology Inc.



Microchip AN874
Driver Theory of Operation
One switching power supply design concept, not
previously mentioned, is the idea of continuous versus
discontinuous inductor current. In a discontinuous
design, the current flow in the inductor drops to zero at
the end of each discharge cycle as previously
discussed. However, in a continuous current design,
the inductor current does not drop to zero. Instead, the
inductor maintains a DC current flow throughout the
switch cycle. The resulting inductor current has both
AC and DC components to its waveform. The DC
component equals the average current flow during the
cycle and is determined by the reference voltage
VDRIVE. The AC component is a triangular shaped
waveform super-imposed on the DC component and is
caused by the switching action of the driver. The
advantage of continuous current design is that the
inductor current flows to the output continuously, which
reduces the charge storing requirements on COUT.
The driver presented in this application note is
designed to take advantage of the DC component of a
continuous inductor current design. See Figure 3 for
the block diagram of the driver. Notice that the circuit is
very similar to the buck topology switching power
supply in Figure 2. The MOSFET (Q1) and Schottky
diode (D) form the switch, the inductor is the same as
the buck topology switching power supply, and the LED
is the load connected to the output. The only
differences are the lack of an output capacitor (COUT),
and feedback is based on the load current instead of
load voltage.
The Buck topology LED driver also has a similar
charge/discharge cycle to the buck topology switching
power supply. The charge phase of the cycle starts by
turning on MOSFET, Q1. This starts an increasing
current flow from the battery, through Q1, the inductor,
the LED, and R8 (see Figure 3). When the current
through R8 generates a voltage greater than the
VDRIVE reference voltage, the comparator turns Q1 off,
ending the charge phase and starting the discharge
phase. During the discharge phase, current flows
through the Diode D, the inductor L1, and R8. The
inductor current ramps down until the current flowing
through R8 generates a voltage less than VDRIVE.
When this occurs, the comparator turns Q1 back on
and the next charge phase begins. The resulting
current flow through the LED/inductor is a DC level with
a small triangular waveform, super-imposed on top,
that is synchronized to the charge/discharge cycle (see
Figure 4).
Because the current through R8 generates a voltage
that toggles about the VDRIVE level, controlling VDRIVE
controls the current output to the LED, making it the
control input to the driver circuit.
AN874
FIGURE 3:
BUCK CONFIGURATION
LED CURRENT CONTROL
VDRIVE
Comparator
VBATT
Q1
P Channel
MOSFET
D
L1
R6
C3
LED
R8
FIGURE 4:
INDUCTOR/LED CURRENT
FLOW
LED
Current
Over shoot
Under shoot
AC
Component
DC Component
Time
off off
Q1
on on
Time
If the driver is built as described above, it will operate at
a frequency determined by the switching speed of the
MOSFET and the propagation delay of the comparator.
Unfortunately, this will switch the MOSFET so rapidly
that the switching time of the MOSFET will be a
significant percentage of the switch time, resulting in
increased switching losses in the transistor. Therefore,
an RC low-pass filter (R6 and C3) is added to the
feedback path to introduce a time delay. The resulting
low-pass filter does two things for the driver circuit:
1. It reduces the amplitude of any fast voltage
transients generated by switching Q1.
2. And, it increases the charge and discharge
periods of the switch cycle, such that the
inductor/LED current will over and undershoot
the VDRIVE level.
© 2006 Microchip Technology Inc.
DS00874C-page 3







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