LED Buck Converter Design Using the IRS2505L

Application Note AN-1214
LED Buck Converter Design Using the IRS2505L
By Ektoras Bakalakos
Table of Contents
Page
1. Introduction ..................................................................................... 2
2. Buck Converter ............................................................................... 2
3. Peak Current Control ...................................................................... 5
4. Zero-Crossing Detection ................................................................. 5
5. IC Start-Up and Supply Circuitry ..................................................... 6
6. Buck LED Design Example 1: Offline Converter ............................. 8
7. Buck LED Design Example 2: DCDC Converter ............................. 9
8. PCB Layout Considerations ............................................................ 11
9. Conclusion ...................................................................................... 13
10. Appendix 1: Reference Design ....................................................... 14
11. References ..................................................................................... 16
12. Revision History .............................................................................. 16
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1.
Introduction
The IRS2505L SOT-23 control IC is a versatile solution for controlling power
supplies in PFC Boost, Flyback, Buck, or Buck-Boost applications. This app note
will cover the use of the IRS2505L in a peak current control Critical Conduction
Mode (CrCM) Buck converter for LED driving. The IC includes all of the
necessary circuitry to control the Buck converter on and off times, regulate the
output current, and protect against over-current fault conditions. During the
design of the Buck circuit, special care should be taken when generating the
circuit schematic, selecting component values and ratings, and generating the
PCB layout. This application note provides detailed design information to help
speed up design time and avoid circuit problems that can occur due to wrong
component values or ratings, incorrect programming of IC parameters, and noise
susceptibility. Helpful information is included for designing the PFC circuit,
designing the IC supply circuitry, and using the IC protection features. PCB
layout guidelines are also included to help avoid noise problems that can cause
circuit malfunction or poor power supply performance. Finally, an excel
spreadsheet design tool (“IRS2505L Buck LED Design Calculator”) [2] is also
included that contains all of the necessary calculations described in this
application note.
2.
Buck Converter
The IRS2505L operates in Critical-Conduction (or transition) Mode for the
Buck converter. The Buck circuit (Figure 1) includes an inductor (LBUCK), a
switch (MBUCK), and a diode (DBUCK). In this Buck configuration, the switch is
referenced to ground and the output is left floating. This enables simple driving of
the switch. During the on-time of the switch, the inductor current ramps up
linearly to a peak value (Figure 2). The peak value depends on the input voltage,
output voltage, inductor value and the on-time. During the off-time of the switch,
the inductor current flows through the diode to the load and discharges back
down linearly to zero. When the current reaches zero, the Buck switch is turned
on again and the cycle repeats.
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Rect (+)
VOUT (+)
COUT
DBUCK
LBUCK
VOUT (-)
CIN
MBUCK
Rect (-)
Figure 1: Buck converter with a floating output.
I LBUCK
I L,PEAK
VIN VOUT
L
 VOUT
L
IL, AVG
t
DT SW
T SW
Figure 2: Inductor current during critical-conduction mode.
To calculate the inductor value as a function of the desired switching frequency,
the following equations can be used. First, the following parameters need to be
defined:
𝑉𝐼𝑁
𝑉𝑂𝑈𝑇
𝜂
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= Input voltage. May be DC or rectified AC. For AC, use the peak
voltage of the line (𝑉𝐴𝐶𝑃𝐾 = 𝑉𝐴𝐶𝑅𝑀𝑆 × √2)
= Output voltage of the load, in this case the LED string.
= Buck converter efficiency (typically 0.85)
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𝑓𝑆𝑊
= Buck switching frequency (occurs at the peak of nominal line
voltage for AC)
𝑇𝑆𝑊
= The switching period (
𝐷
𝐼𝐿,𝐴𝑉𝐺
𝐼𝐿,𝑃𝐸𝐴𝐾
𝐿𝐵𝑈𝐶𝐾
𝐼𝑂𝑈𝑇
𝑃𝑂𝑈𝑇
=
=
=
=
=
=
1
𝑓𝑆𝑊
)
Duty Cycle
Average Buck inductor current
Peak Buck inductor current
Buck Inductor
Output Current of the LED string
Output Power
The peak current of the Buck inductor can be calculated from the slope of the
inductor current from Figure 2, knowing the switching period 𝑇𝑆𝑊 and duty cycle
D of our converter:
𝐼𝐿,𝑃𝐸𝐴𝐾 =
(𝑉𝐼𝑁 −𝑉𝑂𝑈𝑇 )𝐷𝑇𝑆𝑊
[A] [2.1]
𝐿𝐵𝑈𝐶𝐾
Where the duty cycle for the Buck converter is defined as:
𝐷=
𝑉𝑂𝑈𝑇
[2.2]
𝑉𝐼𝑁
For a symmetric triangle as in Figure 2, the average value of the inductor current
is its peak divided by 2:
𝐼𝐿,𝐴𝑉𝐺 =
𝐼𝐿,𝑃𝐸𝐴𝐾
[A] [2.3]
2
We can approximate the average inductor current to equal the output current:
𝐼𝐿,𝐴𝑉𝐺 = 𝐼𝑂𝑈𝑇
[A] [2.4]
Rearranging the equations leads to:
𝐼𝑂𝑈𝑇 =
(𝑉𝐼𝑁 −𝑉𝑂𝑈𝑇 )𝐷𝑇𝑆𝑊
[A] [2.5]
2𝐿𝐵𝑈𝐶𝐾
By selecting the nominal Buck switching frequency 𝑓𝑆𝑊 the only unknown
remaining in equation 2.5 is the buck inductor, which we can solve for:
𝐿𝐵𝑈𝐶𝐾 =
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𝑉
(𝑉𝐼𝑁 −𝑉𝑂𝑈𝑇 )( 𝑂𝑈𝑇 )
𝑉𝐼𝑁
2𝑓𝑆𝑊 𝐼𝑂𝑈𝑇
∗ 106
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[uH] [2.6]
4
3.
Peak Current Control
To regulate the output LED current in a Buck converter, we will use the equations
derived earlier along with the IRS2505L control circuit. Figure 3 shows the control
circuit for the Buck converter.
Rect (+)
VOUT (+)
DBUCK
RVCC1
COUT
LBUCK
VOUT (-)
RVCC2
CMP
CIN
1
COM
2
VCC
3
IRS2505L
CCMP
VBUS
CSN
5
RVCC3
CZX
to VCC
RSN
CVCC2
PFC
RG
4
DVCC
MBUCK
CVCC1
CF
RF
RCS
Rect (-)
Figure 3: IRS2505L Control Circuit for Buck Converter
The VBUS pin over-current threshold for the IRS2505L, VBUSOC+, is used to
regulate the peak current of the converter. At a test condition of VBUS=0V, the
VBUSOC+ typical value is 0.8V.
From the typical control circuit in Figure 3, the RCS resistor programs the peak
inductor current for peak current control during the on-time of the switch, which is
fed back to the VBUS pin through a low pass filter, RF and CF, filtering out
unwanted switching noise.
The value of RCS that gives the desired output LED current can be calculated by
rearranging equations 2.3-2.4 with Ohm’s law:
𝑅𝐶𝑆 =
4.
0.8𝑉
[Ohms] [3.1]
2𝐼𝑂𝑈𝑇
Zero-Crossing Detection
The zero-crossing detection of the PFC inductor current utilizes the gate drive pin
(PFC) together with the drain-to-gate capacitance of the external Buck MOSFET.
An additional capacitor, CZX, in Figure 3 is used to couple the zero crossing
information of the Buck inductor to the PFC pin to ensure proper detection in the
Buck topology. During the on-time, the gate drive pulls the gate of the external
Buck MOSFET up to VCC and turns the MOSFET on. The inductor ramps up to
a peak level (Figure 4). When the on-time ends, the gate is pulled to COM for a
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short delay and then increased and held at a given offset voltage, VPFCOFF
(0.6V, typical). When the Buck inductor current discharges to zero, the drain-gate
capacitance with the additional help of CZX “pulls” the gate signal below the
zero-crossing reset threshold, VPFCZX- (0.4V, typical), at the PFC pin and the
gate turns on again. This new and innovative method from IR is simple and does
not require a secondary winding from the inductor to detect zero-crossings.
A typical value for the CZX capacitor is 22pF.
GATE
VCC
VPFCOFF
VPFCZX-
t
ILBUCK
t
VDRAIN
DC BUS
t
OFF-TIME
ON-TIME
Figure 4: Gate voltage (upper trace), Buck inductor current (middle trace), and
MOSFET drain voltage (lower trace) during normal on- and off-time switching
period.
5.
IC Start-Up and Supply Circuitry
The external IC supply circuit is designed to perform two main functions:
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1)
2)
Supply the start-up and stand-by current to VCC.
Supply the necessary ICC current to VCC during all operating modes.
The start-up current to VCC is supplied by the start-up resistors, RVCC1 and
RVCC2, connected between the rectified input voltage and VCC (Figure 3). Two
resistors are used in order to properly withstand the high voltage between the
rectified BUS and VCC.
The values for resistors RVCC1 and RVCC2 are calculated using the desired
VCC start-up time (𝑡𝑆𝑇𝐴𝑅𝑇 ), the VCC capacitor value (𝐶𝑉𝐶𝐶1 ), the IC rising
VCCUV+ turn-on threshold (11.1V, typical) and the minimum rectified input
voltage (𝑉𝐼𝑁𝑀𝐼𝑁 ). The resistor values are calculated as:
𝑅𝑉𝐶𝐶1 = 𝑅𝑉𝐶𝐶2 ≅
−𝑡𝑆𝑇𝐴𝑅𝑇
2𝐶𝑉𝐶𝐶1 ln(1−
𝑉𝐶𝐶𝑈𝑉+
)
𝑉𝐼𝑁𝑀𝐼𝑁
[sec] [5.1]
The maximum power loss for resistors RVCC1 and RVCC2 occurs when the
rectified input voltage is at the maximum value of the specified input voltage
range. The power loss in each transistor is calculated as:
𝑃𝑅𝑉𝐶𝐶1 = 𝑃𝑅𝑉𝐶𝐶2 ≅
(𝑉𝐼𝑁𝑀𝐴𝑋 −𝑉𝐶𝐶)2
2(𝑅𝑉𝐶𝐶1+𝑅𝑉𝐶𝐶2 )
[Watts] [5.2]
The power loss and resulting temperature of RVCC1 and RVCC2 should be
measured on the bench under high input voltage conditions to make sure the
power rating of the resistors is adequate.
When the input voltage is first applied to the circuit, VCC ramps up with a time
constant given by RVCC1, RVCC2 and CVCC1 (see Figure 5). After VCC
exceeds VCCUV+, the IC turns on and the gate driver output (PFC pin) begins
oscillating. The PFC pin turns the external Buck MOSFET on and off causing the
drain node to switch between the rectified input and COM. The auxiliary VCC
supply circuit (CSN, RSN, DVCC, CVCC2, RVCC3) then takes over as the main
supply circuit for the IC and VCC increases up to the clamp voltage of the
external Zener diode on VCC (not shown, typically 14-16V).
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Figure 5: VCC (red) and PFC pin gate voltage (yellow) during normal start-up
conditions.
6.
Buck LED Design Example 1: Offline Converter
All of the necessary design calculations have been included inside the excel
spreadsheet calculation tool (“IRS2505L Buck LED Design Calculator”) [2] that
accompanies this application note. The following design example calculations are
for a 9W LED driver with an offline 120VAC nominal input (+/-10%). Please use
the “AC” tab in the excel tool for this type of design. The input parameters for the
circuit are determined as the following:
𝑉𝐴𝐶,𝑁𝑂𝑀
𝑉𝑂𝑈𝑇
𝐼𝑂𝑈𝑇
𝑓𝑆𝑊,𝑁𝑂𝑀
=
=
=
=
120VAC
25V
350mA
100kHz
These values are input into the yellow fields of the “User Input
Parameters” section of the spreadsheet as follows:
User Input Parameters
Parameter
VAC_nom
VOUT_max
IOUT_nom
POUT
FSW_nom
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User Input Value
120
25
0.35
8.75
100
Units
Vrms
VDC
A
W
kHz
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Description
Nominal r.m.s. input voltage
Max Output LED String Voltage
Nominal Output LED Current
Output power
Nominal Switching Frequency
8
The buck circuit calculations are then given in the green sections of the “Buck
Circuit Calculations” section of the spreadsheet as follows:
Buck Circuit Calculations
Parameter
I_LBUCK_peak
LBUCK
Calculated Value
0.7
304
Units
Apk
uH
Description
Maximum peak inductor current
Buck inductor value
The IRS2505L programming components are then calculated in the green
sections of the “IRS2505L Programming Components” section using the yellow
fields as the user inputs. The red sections are fixed and typically should remain
fixed for different designs.
IRS2505L Programming Components
Calculated Value
10
0.1
120
120
10
231
Units
uF
uF
k Ohms
k Ohms
Ohms
msec
RCS
1.14
Ohms
RG
CSN
RSN
CZX
RF
CF
CCMP
22
220
10
22
1.0
100
10.0
Ohms
pF
Ohms
pF
k Ohms
pF
nF
Parameter
CVCC1
CVCC2
RVCC1
RVCC2
RVCC3
t_startup
Description
VCC capacitor value (user input value)
VCC filter capacitor value (fixed)
VCC start-up resistor no. 1 (user input)
VCC start-up resistor no. 2 (user input)
VCC limit resistor (user input)
VCC start-up time at VAC_min
Peak-current programming resistor value
Gate resistor value (user input)
VCC charging capacitor (user input)
VCC charging resistor (user input)
ZX coupling capacitor (fixed)
Current-sense filter resistor value (fixed)
Current-sense filter capacitor value (fixed)
CMP pin compensation capacitor value (fixed)
The corresponding circuit (Figure 3) for all of these calculations is also given
inside the spreadsheet.
7.
Buck LED Design Example 2: DCDC Converter
All of the necessary design calculations have been included inside the excel
spreadsheet calculation tool (“IRS2505L Buck LED Design Calculator”) [2] that
accompanies this application note. The following design example calculations are
for a 9W LED driver with a DC input voltage varying from 40-60V. Please use the
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“DC” tab in the excel tool for this type of design. The input parameters for the
circuit are determined as the following:
𝑉𝐼𝑁,𝑀𝐼𝑁
𝑉𝐼𝑁,𝑀𝐴𝑋
𝑉𝑂𝑈𝑇
𝐼𝑂𝑈𝑇
𝑓𝑆𝑊,𝑀𝐼𝑁
=
=
=
=
=
40V
60V
25V
350mA
100kHz
These values are input into the yellow fields of the “User Input Parameters”
section of the spreadsheet as follows:
User Input Parameters
Parameter
VIN_min
VIN_max
VOUT_max
IOUT_nom
POUT
FSW_min
User Input Value
40
60
25
0.35
8.75
100
Units
VDC
VDC
VDC
A
W
kHz
Description
Min DC input voltage
Max DC input voltage
Max Output LED String Voltage
Nominal Output LED Current
Output power
Minimum Switching Frequency
The buck circuit calculations are then given in the green sections of the “Buck
Circuit Calculations” section of the spreadsheet as follows:
Buck Circuit Calculations
Parameter
I_LBUCK_peak
LBUCK
FSW_max
Calculated Value
0.7
134
156
Units
Apk
uH
kHz
Description
Maximum peak inductor current
Buck inductor value
Maximum Switching Frequency
The IRS2505L programming components are then calculated in the green
sections of the “IRS2505L Programming Components” section using the yellow
fields as the user inputs. The red sections are fixed and typically should remain
fixed for different designs.
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IRS2505L Programming Components
Parameter
CVCC1
CVCC2
RVCC1
RVCC2
RVCC3
t_startup
Calculated Value
10
0.1
47
47
10
302
Units
uF
uF
k Ohms
k Ohms
Ohms
msec
RCS
1.14
Ohms
RG
CSN
RSN
CZX
RF
CF
CCMP
22
220
10
22
1.0
100
10.0
Ohms
pF
Ohms
pF
k Ohms
pF
nF
Description
VCC capacitor value (user input value)
VCC filter capacitor value (fixed)
VCC start-up resistor no. 1 (user input)
VCC start-up resistor no. 2 (user input)
VCC limit resistor (user input)
VCC start-up time at VIN_min
Peak-current programming resistor value
Gate resistor value (user input)
VCC charging capacitor (user input)
VCC charging resistor (user input)
ZX coupling capacitor (fixed)
Current-sense filter resistor value (fixed)
Current-sense filter capacitor value (fixed)
CMP pin compensation capacitor value (fixed)
The corresponding circuit (Figure 3) for all of these calculations is also given
inside the spreadsheet.
8.
PCB Layout Considerations
For correct circuit functionality and to avoid high-frequency noise problems,
proper care should be taken when designing the PCB layout. Typical design
problems due to poor layout can include high-frequency voltage and/or current
spikes, EMC issues, latch up, abnormal circuit behavior, component failures, low
manufacturing yields, and poor reliability. The following layout tips should be
followed as early in the design phase as possible in order to reduce circuit
problems, shorten design cycles, and to increase reliability and manufacturability:
1) Keep high-frequency, high-current traces as short as possible (drain
switching node, output diode node). This will help reduce noise due to
parasitic inductance of PCB traces.
2) Keep high-frequency switching nodes away from quiet or critical circuit
nodes (CMP pin, VBUS pin). This will help reduce noise coupling from
switching nodes to other circuit nodes.
3) Place high-frequency filter capacitors directly at their IC pins (VCC pin).
This will help insure the best possible filtering against high-frequency
noise.
4) Do not connect power ground through IC ground or small-signal filter or
programming component ground. Keep separate traces for power and IC
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5)
6)
7)
8)
9)
or small-signal grounds and connect small-signal ground to power ground
at a single point only. This will prevent high-frequency noise from
occurring on critical small-signal nodes or IC pins which can cause circuit
malfunction or failures.
Reduce the distance of the power switches to their gate drive pins as
much as possible (PFC). This will help reduce the parasitic inductance in
the traces. This will reduce possible voltage spikes due to gate drive
switching and help prevent latch up due to voltage over- or under-shoot.
Use a limiting resistor in between the auxiliary supply and VCC. This will
help prevent damage due to high-voltage or high-current spikes from the
charge pump supply that can cause electrical overstress of the IC.
Place critical sensing nodes (current-sensing resistor, ZX detection) as
close to the IC as possible. This will help eliminate false triggering or
circuit malfunction due to noise being coupled onto to sensitive control
signals.
Check inductor for saturation. Saturation of the inductor results in currents
with very high di/dt levels. These high di/dt signals can induce noise
everywhere in the circuit and cause many different noise related issues.
Make sure the inductor is properly designed to handle the maximum peak
currents under all operating conditions.
See Figure 6 for PCB layout guidelines around the IRS2505L.
IC and Power
Ground
Connect at
Single Point
Only!
Power
Ground
Current Sensing
Component
VCC Filter
Capacitor
CMP Pin
Capacitor IC Ground
IRS2505L
Gate Drive
Trace
MOSFET
Drain
Switching
Node
Figure 6: PCB Layout Guidelines
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9.
Conclusion
The information presented in this application note will help improve the
design of the LED Buck converter and help reduce potential circuit problems.
Ease of using and programming the IC, correct design of the Buck stage, design
of the IC supply, and proper PCB layout guidelines help minimize design time,
maximize performance, and maximize manufacturability and robustness of the
final design. Finally, an excel spreadsheet design tool (“IRS2505L Buck LED
Design Calculator”) is also available that contains all of the necessary
calculations described in this application note.
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10. Appendix I: Reference Design (Schematic/BOM)
a. Schematic (VIN = 100-120VAC, VOUT = 28V, IOUT = 315mA)
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b. BOM (VIN = 100-120VAC, VOUT = 28V, IOUT = 315mA)
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11. References
[1] IRS2505L SOT-23 Boost PFC Control IC Datasheet
[2] IRS2505L Buck LED Design Calculator
12. Revision History
Date
01/09/2015
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Revision
1
Changes
Initial version
AN-1214
Author
Ektoras Bakalakos
16