Texas Instruments Incorporated AN-1967 User's Guide

VIN (V)
EFFICIENCY (%)
100
95 90 85 80 75 70
0 16 32 48 64 80

1 Introduction

This wide range evaluation board showcases the LM3424 NFET controller used with a buck-boost current regulator. It is designed to drive 4 to 10 LEDs at a maximum average LED current of 1A from a DC input voltage of 10 to 70V.
The evaluation board showcases many of the LM3424 features including thermal foldback, analog dimming, external switching frequency synchronization, and high frequency PWM dimming, among others. There are many external connection points to facilitate the full evaluation of the LM3424 device including inputs, outputs and test points. Refer to Table 1 for a summary of the connectors and test points.
The buck-boost circuit can be easily redesigned for different specifications by changing only a few components (see the Alternate Designs section). Note that design modifications can change the system efficiency for better or worse.
This application note is designed to be used in conjunction with the LM3424 datasheet as a reference for the LM3424 buck-boost evaluation board. Refer to the LM3424 Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs (SNVS603) data sheet for a comprehensive explanation of the device, design procedures, and application information.
SNVA397A–August 2009–Revised May 2013
AN-1967 LM3424 Buck-Boost Evaluation Board

2 Key Features

Input: 10V to 70V
Output: 4 to 10 LEDs at 1A
Thermal Foldback / Analog Dimming
PWM Dimming up to 10 kHz
External Synchronization > 500 kHz
Input Under-voltage and Output Over-voltage Protection
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Figure 1. Efficiency with 9 Series LEDS AT 1A
1
Copyright © 2009–2013, Texas Instruments Incorporated
External Connection Descriptions

3 External Connection Descriptions

Table 1. Connectors and Test Points
Qty Name Description Application Information
J1 V
IN
J2 GND Input Ground Connect to negative terminal of supply voltage (GND). J3 EN Enable On/Off Jumper connected enables device. J4 LED+ LED Positive Connect to anode (top) of LED string. J5 LED- LED Negative Connect to cathode (bottom) of LED string. J6 BNC Dimming Input Connect a 3V to 10V PWM input signal up to 10 kHz for PWM dimming the LED load. J7 OUT Output with NTC Alternative connector for LED+ and LED-. Pins 4 and 11 are used for connecting an
TP1 SW Switch Node Test point for switch node (where Q1, D1, and L1 connect).
TP3 SGND Signal Ground Connection for GND when applying signals to TP5, TP8, and TP9. TP4 LED+ LED Positive Test point for anode (top) of LED string.
TP5 nDIM Inverted Dim Signal Test point for dimming input (inverted from input signal). TP6 V
IN
TP8 SYNC Synchronization Connect a 3V to 6V PWM clock signal > 500 kHz (pulse width of 100ns) to synchronize
TP9 NTC Temp Sense Input Connect a 0V to 1.24V DC voltage to analog dim the LED current. TP10 PGND Power Ground Test point for GND when monitoring TP1, TP4, or TP6.
Input Voltage Connect to positive terminal of supply voltage.
external NTC thermistor. Refer to schematic for detailed connectivity.
Voltage
Voltage
Input Voltage Test point for input voltage.
Input the LM3424 switching frequency to the external clock.
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SS
TGAIN
OVP
LM3424
nDIM
GND
TSENSE
TREF
DDRV
VS
DAP
GATE
EN
COMP
VIN
CSH
RT/SYNC
IS
HSN
SLOPE
VCC
HSP
Q7
D1
L1
R14
C9
R6
Q1
C7
R17
R11
R13
R5
R15
NTC
R19
R21
R22
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
R8
R7
C22
R9
R20
C12
C11
C15
Q3
PWM
R4
Q6
Q5
Q4
D2
V
IN
LED+
V
IN
DIM
DIM
R24
R23
Q2
C4, C5, C6, C17, C19
C2, C3, C16, C18, C23
C1
R3
J3
R12
J6
TP5
R2
C8
R1
R10
3
4
5
C10
C13
C14
R25
TP3 TP10
TP8
TP6
TP1
TP4
V
IN
J1
GND
J2
R26
C20
C21
8 9
10
12 13 14
7 6 5
3 2 1
J7
11 4
LED-
J4
J5
NTC
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4 Schematic

Schematic
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3
LM3424 Pin Descriptions

5 LM3424 Pin Descriptions

Pin Name Description Application Information
1 V
2 EN Enable the device or to < 0.8V for low
3 COMP Compensation
4 CSH Current Sense High
5 RT Resistor Timing synchronize external clock as
6 nDIM Not DIM input
7 SS Soft-start
8 TGAIN Temperature Foldback Gain
9 TSENSE Temperature Sense Input
10 TREF VSto set the temperature
11 V
12 OVP Over-Voltage Protection Turn-off threshold is 1.24V and
13 DDRV Dimming Gate Drive Output
14 GND Ground
15 GATE Gate Drive Output
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Bypass with 100 nF capacitor
IN
Input Voltage
to GND as close to the device as possible in the circuit board layout.
Connect to > 2.4V to enable power shutdown.
Connect a capacitor to GND to compensate control loop.
Connect a resistor to GND to set the signal current. Can also be used to analog dim as explained in the Thermal
Foldback / Analog Dimming
section of the datasheet. Connect a resistor to GND to
set the switching frequency. Can also be used to
explained in the Switching Frequency section of the datasheet.
Connect a PWM signal for dimming as detailed in the PWM Dimming section of the datasheet and/or a resistor divider from VINto program input under-voltage lockout (UVLO). Turn-on threshold is
1.24V and hysteresis for turn­off is provided by 20 µA current source.
Connect a capacitor to GND to extend start-up time.
Connect a resistor to GND to set the foldback slope.
Connect a resistor/ thermistor divider from VSto sense the temperature as explained in the Thermal Foldback / Analog Dimming section of the datasheet.
Temperature Foldback
Reference
Connect a resistor divider from foldback reference voltage.
2.45V reference for
S
Voltage Reference temperature foldback circuit
and other external circuitry. Connect to a resistor divider
from VOto program output over-voltage lockout (OVLO).
hysteresis for turn-on is provided by 20 µA current source.
Connect to gate of dimming MosFET.
Connect to DAP to provide proper system GND
Connect to gate of main switching MosFET.
4
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Pin Name Description Application Information
16 V
17 IS Main Switch Current Sense switch for R
18 SLOPE Slope Compensation
19 HSN resistor to the negative side of
20 HSP resistor to the positive side of
DAP (21) DAP Thermal pad on bottom of IC 9 vias to bottom layer ground

6 Bill of Materials

Qty Part ID Part Value Manufacturer Part Number
4 C1, C5, C20, C23 0.1 µF X7R 10% 100V TDK C2012X7R2A104K 4 C2, C3, C16, C18 4.7 µF X7R 10% 100V TDK C5750X7R2A475K 4 C4, C6, C17, C19 10 µF X7R 10% 50V TDK C5750X7R1H106K 2 C7, C22 0.47 µF X7R 10% 16V MURATA GRM21BR71C474KA01L 0 C8 DNP 1 C9 2.2 µF X7R 10% 16V MURATA GRM21BR71C225KA12L 1 C10 1 µF X7R 10% 16V MURATA GRM21BR71C105KA01L 1 C11 47 pF COG/NPO 5% 50V AVX 08055A470JAT2A 1 C12 0.22 µF X7R 10% 16V MURATA GRM219R71C224KA01D 3 C13, C14, C21 100 pF COG/NPO 5% 50V MURATA GRM2165C1H101JA01D 1 C15 1 µF X7R 10% 16V MURATA GRM21BR71C105MA01L 1 D1 Schottky 100V 12A VISHAY 12CWQ10FNPBF 1 D2 Zener 10V ON-SEMI BZX84C10LT1G 4 J1, J2, J4, J5 Banana Jack KEYSTONE 575-8 1 J3 1x2 Header Male SAMTEC TSW-102-07-T-S 1 J6 BNC connector AMPHENOL 112536 1 J7 2x7 Header Male Shrouded SAMTEC TSSH-107-01-SDRA
1 L1 33 µH 20% 6.3A COILCRAFT MSS1278-333MLB 2 Q1, Q2 NMOS 100V 32A FAIRCHILD FDD3682 1 Q3 NMOS 60V 260mA ON-SEMI 2N7002ET1G 1 Q4 PNP 40V 200mA FAIRCHILD MMBT5087 1 Q5 PNP 150V 600 mA FAIRCHILD MMBT5401 1 Q6 NPN 300V 600mA FAIRCHILD MMBTA42 1 Q7 NPN 40V 200mA FAIRCHILD MMBT6428 2 R1, R11 12.4 k1% VISHAY CRCW080512K4FKEA 0 R2 DNP 3 R3, R20, R26 101% VISHAY CRCW080510R0FKEA 1 R4 17.4 k1% VISHAY CRCW080517K4FKEA 1 R5 1.43 k1% VISHAY CRCW08051K43FKEA
CC
RA
Internal Regulator Output
High-Side LED Current Sense
Negative
High-Side LED Current Sense
Positive
Bill of Materials
Bypass with a 2.2 µF–3.3 µF, ceramic capacitor to GND.
Connect to the drain of the main N-channel MosFET
a sense resistor installed in the source of the same device.
Connect a resistor to GND to set slope of additional ramp.
Connect through a series the LED current sense resistor.
Connect through a series the LED current sense resistor.
Connect to GND and place 6 ­pour.
sensing or to
DS-ON
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Bill of Materials
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1 R6 0.041% 1W VISHAY WSL2512R0400FEA 2 R7, R8 1.0 k1% VISHAY CRCW08051K00FKEA 1 R9 0.11% 1W VISHAY WSL2512R1000FEA 1 R10 14.3 k1% VISHAY CRCW080514K3FKEA 4 R12, R13, R14, R15 10.0 k1% VISHAY CRCW080510K0FKEA 1 R17 499 k1% VISHAY CRCW0805499KFKEA 3 R19, R21, R22 49.9 k1% VISHAY CRCW080549K9FKEA 1 R23 4991% VISHAY CRCW0805499RFKEA 1 R24 4.99 k1% VISHAY CRCW08054K99FKEA 1 R25 1501% VISHAY CRCW0805150RFKEA 8 TP1, TP3, TP4, Turret Keystone 1502-2
TP5, TP6, TP8, TP9, TP10
1 U1 Buck-boost controller TI LM3424
6
AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013
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7 PCB Layout

PCB Layout
Figure 2. Top Layer
Figure 3. Bottom Layer
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R10 = =
= 14.4 k:
1 + 1.95e-8 x f
SW
1.40e
-10
x f
SW
1 + 1.95e-8 x 500 kHz
1.40e
-10
x 500 kHz
D
MAX
==
677.0
=
V21
V10V21 +
V
O
VV
IN-MINO
+
21V
=
21V + 70V
= 0.231
D
MIN
=
VO + V
IN-MAX
V
O
533.0467.01D1'D
=
-
=
-
=
D
==
467.0
=
V21
V24V21 +
V
O
VV
INO
+
:
=
:x
=
x
=
95.1m3256rNr
LEDD
V21V5.36VNV
LEDO
=
x
=
x
=
Design Procedure

8 Design Procedure

8.1 Specifications

N = 6 V
= 3.5V
LED
r
= 325 m
LED
VIN= 24V V
= 10V
IN-MIN
V fSW= 500 kHz V I
Δi Δi Δv
I V V V V TBK= 45°C T t
IN-MAX
SNS
= 1A
LED
L-PP
LED-PP
IN-PP
= 6A
LIM
TURN-ON
HYS
TURN-OFF
HYSO
= 125°C
END
= 40 ms
TSU
= 70V
= 100 mV
= 700 mA
= 12 mA
= 100 mV
= 10V
= 3V
= 50V
= 10V
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8.2 Operating Point

8.3 Switching Frequency

8
Solve for VOand rD:
(1) (2)
Solve for D, D', D
MAX
, and D
MIN
:
(3) (4)
(5)
(6)
Solve for RT:
(7)
AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013
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PP-==L
DVINx
f1LSWx
kHz045H33 xP
467.0V42 x
mA674
=
'i
==
DVINx
fSWx
467.0V42 x
PH
32
=
1L
PP-'iL
kHz045700 mA x
R15 = 10 k: R19 = R21 = R22 = 49.9 k:
R
GAIN
=
R19
x 2.45V
100 PA
R19 + R21
-
¸
¸ ¹
·
¸ ¸
¹
·
R
NTC-END
R
NTC-END
+ R22
I
CSH
= 9.49 k:
R
GAIN
=
1
x 2.45V
2
-
¸
¸ ¹
·
¸ ¸
¹
·
6.34 k: + 49.9 k:
6.34 k:
R8 = R7 = 1 k:
R1 = 12.4 k:
R9 = 0.1:
LED
=
= = 1.0A
1.24V x R8 R9 x R1
1.24V x 1.0 k:
0.1: x 1.24 k:
= 1.0 k:
I
LED
x R1 x R9
1.24V
1A x 1.24 k: x 0.1:
= 0.1:
V
SNS
100 mV
1A
I
LED
R10 = 14.3 k:
fSW =
= 504 kHz
1.40e
-10
x 14.3 k: - 1.95e
-8
1
fSW =
1.40e
-10
x R10 - 1.95e
-8
1
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The closest standard resistor is 14.3 ktherefore fSWis:
The chosen component from step 2 is:

8.4 Average LED Current

Design Procedure
(8)
(9)
Solve for R
Assume R
:
SNS
= 12.4 kand solve for R
CSH
The closest standard resistor for R
The chosen components from step 3 are:

8.5 Thermal Foldback

Using a standard 100k NTC thermistor (connected to pins 4 and 11 of J7), find the resistances corresponding to TBKand T datasheet. Assuming R
Solve for R
GAIN
:
END(RNTC-BK
= R
REF1
:
HSP
is actually 0.1and for R
SNS
= 243 kand R
= 49.9 k, then R
REF2
NTC-END
BIAS
is actually 1 ktherefore I
HSP
= 71.5 k) from the manufacturer's
= R
NTC-BK
= 243 k.
LED
(10)
(11)
is:
(12)
(13)
The chosen components from step 4 are:

8.6 Inductor Ripple Current

Solve for L1:
The closest standard inductor is 33 µH therefore Δi
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is:
L-PP
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(14)
(15)
(16)
(17)
9
= 9.9 k:
R15 =
VO x R10 x R9
R15 =
1.5e13 x L1
35V x 14.3 k: x 0.1:
1.5e13 x 33 PH
R6 = 0.04:
=
0.04:
= 6.13A
I
LIM
=
R6
245 mV 245 mV
=
6A
= 0.041:
R6 =
I
LIM
245 mV 245 mV
C4 = C6 = C17 = C19 = 10 PF
x
A1
=
I
LED
I
RMSCO-
=
1- 0.677
677.0
1.45A
x
1- D
MAX
D
MAX
=
DI
LED
x
=
'i
PP-LED
SW
fxrDx
C
O
2
= =
kHz04595.1 xx:
1 mA
467.0A1 x
F40 P
'i
PP-LED
f
'i
r
SWPP-LEDD
xx
DI
LED
x
CO=
2
= F39.6P=
kHz045mA195.1 xx:
467.0A1 x
C
O
H331L P
=
A89.1
12
1
1
I
RMSL
=
+
x
=
-
I
I
LED
RMSL
x
=
-
12
1
1
2
x
+
¸
¸ ¹
·
¨
¨ ©
§
Di
PPL
c
x
'
-
I
LED
D
c
533.0mA674
2
x
¸
¸ ¹
·
¨
¨ ©
§
A1
x
533.0
A1
Design Procedure
Determine minimum allowable RMS current rating:
The chosen component from step 5 is:

8.7 Output Capacitance

Solve for CO:
The closest capacitance totals 40 µF therefore Δi
LED-PP
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(18)
(19)
(20)
is:
Determine minimum allowable RMS current rating:
The chosen components from step 6 are:

8.8 Peak Current Limit

Solve for R
The closest standard resistor is 0.04 therefore I
The chosen component from step 7 is:
LIM
:

8.9 Slope Compensation

Solve for R
SLP
:
LIM
(21)
(22)
(23)
(24)
is:
(25)
(26)
The chosen component from step 8 is:
10
AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013
(27)
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C2 = C3 = C16 = C18 = 4.7 PF
x
A1
=
I
LED
I
RMSIN-
=
1- 0.677
677.0
1.45A
x
1- D
MAX
D
MAX
=
C
IN
==
kHz504mV100 x
467.0A1 x F27.9 P=
f
'v
SWPPIN-
x
DI
LED
x
C12 = 0.22 PF
C10 = 1 PF
R20 = 10:
C12 = =
= 0.28 PF
1
10:xZP3
1
rad
sec
10:x 360k
1010(max
1P
xZ
=
x
=
,
1Z1P
ZZ
sec
rad
k36
sec
rad
k36010=x
=
3P
Z
3P
Z
)
C10 = =
= 0.30 PF
1
Z
P2
x 5e
6
:
1
0.675 x 5e
6
:
rad sec
= =
sec
rad
675.0=
sec
rad
k19
56305 x56305x
1Z
Z
2P
=Z
),min(
1Z1P
ZZ
T50Ux
TU0 =
= = 5630
D' x 620V
(1 + D) x I
LED
x R6
0.533 x 620V
1.467 x 1A x 0.04:
sec
rad
k36===
533.095.12x: H33467.0 Px
Dr
2
D
c
x
L1Dx
1Z
Z
sec
rad
k19===
1.467 F40
1.95:
Px
COrDx
D1+
1P
Z
R
SLP
= 10 k:
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8.10 Loop Compensation

ω
is approximated:
P1
ω
is approximated:
Z1
TU0is approximated:
To ensure stability, calculate ωP2:
Solve for C
CMP
:
Design Procedure
(28)
(29)
(30)
(31)
(32)
To attenuate switching noise, calculate ωP3:
Assume RFS= 10and solve for CFS:
The chosen components from step 9 are:

8.11 Input Capacitance

Solve for the minimum CIN:
To minimize power supply interaction a 200% larger capacitance of approximately 20 µF is used, therefore the actual Δv capacitors are chosen.
Determine minimum allowable RMS current rating:
is much lower. Since high voltage ceramic capacitor selection is limited, four 4.7 µF X7R
IN-PP
(33)
(34)
(35)
(36)
(37)
The chosen components from step 10 are:
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(38)
(39)
11
Copyright © 2009–2013, Texas Instruments Incorporated
V
TURN-ON
=
R5
1.24V x (1.43 k: + 10 k:)
1.43 k:
1.24V x (R5 + R13)
= 9.91V
V
TURN-ON
=
1.24V x R13
V
TURN-ON
- 1.24V
= 1.42 k:
R5 = =
1.24V x 10 k: 10V - 1.24V
HYS
=
R5
20 PA x 17.4 k: x (1.43 k: + 10 k:)
1.43 k:
HYS
=
+ 20 PA x R13
20 PA x R4 x (R5 + R13)
+ 20 PA x 10 k: = 2.98V
R4 =
R5 x (V
HYS
- 20 PA x R13)
20 PA x (R5 + R13)
= 17.5 k:
R4 =
1.43 k: x (3V - 20 PA x 10 k:) 20 PA x (1.43 k: + 10 k:)
D1 o 12A, 100V, DPAK
mW600mV600A1VIP
FDDD
=
x
=
x
=
A1II
LEDMAXD==-
V91V21V70VVV
OMAXINMAXRD
=
+
=
+
=
--
Q1 o 32A, 100V, DPAK
mW82m50A28.1RIP
2
DSON
2
RMSTT
=
:x
=
x
=
-
x
I
RMST=-
I
LED
D
c
=
x
A28.1
=
0.467
A1
533.0
D
=
A2.1A1=x
677.01-
677.0
I
MAXT-
V91V21V70VVV
OMAXINMAXT
=+=+=
--
Design Procedure

8.12 NFET

Determine minimum Q1 voltage rating and current rating:
A 100V NFET is chosen with a current rating of 32A due to the low R PT:
The chosen component from step 11 is:

8.13 Diode

Determine minimum D1 voltage rating and current rating:
= 50 m. Determine I
DS-ON
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and
T-RMS
(40)
(41)
(42)
(43)
(44)
(45)
A 100V diode is chosen with a current rating of 12A and VD= 600 mV. Determine PD:
The chosen component from step 12 is:

8.14 Input UVLO

Solve for R
The closest standard resistor is 150 ktherefore V
Solve for R
UV2
UV1
(46)
(47)
(48)
:
(49)
is:
HYS
(50)
:
The closest standard resistor is 21 kmaking V
12
AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013
(51)
:
TURN-ON
(52)
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CSS = 0.47 PF
CSS =
= = 540 nF
(40 ms - 29.2 ms)
(t
TSU
- t
SU-SS-BASE
)
20 k: 20 k:
V
O
I
LED
t
SU-SS-BASE
= 168: x C9 + 28 k:x C10 + x C
O
t
SU-SS-BASE
= 29.2 ms
21V
1A
t
SU-SS-BASE
= 168: x 2.2 PF + 28 k:x 1.0 PF + x 40 PF
V
O
I
LED
tSU = 168: x C9 + 36 k:x C10 + x C
O
tSU = 37.2 ms
21V
1A
tSU = 168: x 2.2 PF + 36 k:x 1.0 PF + x 40 PF
R11 = 12.4 k: R17 = 499 k:
= 51.1V
12.4 k:
1.24V x (12.4 k: + 499 k:)
V
TURN-OFF
=
R11
1.24V x (R11 + R17)
V
TURN-OFF
=
R11 = =
= 12.5 k:
V
TURN-OFF
- 1.24V
1.24V x R17
1.24V x 499 k: 50V - 620 mV
V
HYSO
= R17 x 20 PA = 499 k: x 20 PA = 9.98V
R17 =
=
= 500 k:
V
HYSO
10V
20 PA20 PA
R4 = 17.4 k:
R13 = 10 k:
R5 = 1.43 k:
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The chosen components from step 13 are:

8.15 Output OVLO

Design Procedure
(53)
Solve for R
The closest standard resistor is 499 ktherefore V
Solve for R
The closest standard resistor is 15.8 kmaking V
The chosen components from step 14 are:

8.16 Soft-Start

Solve for tSU:
OV2
OV1
:
(54)
is:
HYSO
(55)
:
(56)
TURN-OFF
:
(57)
(58)
If tSUis less than t
, solve for t
TSU
Solve for CSS:
The chosen component from step 15 is:
SNVA397A–August 2009–Revised May 2013 AN-1967 LM3424 Buck-Boost Evaluation Board
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SU-SS-BASE
Copyright © 2009–2013, Texas Instruments Incorporated
:
(59)
(60)
(61)
(62)
13
I
LED
(A)
V
SW
(V)
60
40
20
0
1.0
0.5
0.0
I
LED
2 Ps/DIV
V
SW
I
LED
(A)
V
DIM
(V)
10
5
0
1.0
0.0
-1.0
I
LED
V
DIM
4 ms/DIV
Typical Waveforms

9 Typical Waveforms

TA= +25°C, VIN= 24V and VO= 21V.
Figure 4. Standard Operation Figure 5. 200Hz 50% PWM Dimming
TP1 Switch Node Voltage (VSW) TP5 Dim Voltage (V
LED Current (I
) LED Current (I
LED
LED
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)
DIM
)

10 Alternate Designs

Alternate designs with the LM3429 evaluation board are possible with very few changes to the existing hardware. The evaluation board FETs and diodes are already rated higher than necessary for design flexibility. The input UVLO, output OVP, input and output capacitance can remain the same for the designs shown below. These alternate designs can be evaluated by changing only R9, R10, and L1.
Table 2 gives the main specifications for four different designs and the corresponding values for R9, R10,
and L1. PWM dimming can be evaluated with any of these designs.
Specification / Design 1 Design 2 Design 3 Design 4
Component
V
IN
V
O
f
SW
I
LED
R9 0.05 0.2 0.04 0.08
R10 12.1 k 10.2 k 14.3 k 10.2 k
L1 22µH 68µH 15µH 33µH
Table 2. Alternate Design Specifications
10V - 45V 15V - 50V 20V - 55V 25V - 60V
14V 21V 28V 35V
600kHz 700kHz 500kHz 700kHz
2A 500mA 2.5A 1.25A
14
AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013
Copyright © 2009–2013, Texas Instruments Incorporated
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