background image

V

O

 x I

LED

V

IN

 x I

IN

©

§

¹

·

= 0.85

©

§

¹

·

P

IN

P

O

K

f

SW

 =

25

C

1

 x R

1

©

§

¹

·

= 588 kHz

D =

t

ON

t

ON

 + t

OFF

©

§

¹

·

= t

ON

 x f

SW

|

D

V

O

V

IN

 + V

O

©

§

¹

·

|

D

D

©

§

¹

·

V

IN

V

O

©

§

¹

·

t

S

'

T

D

S

DT

L

Â

i

L

i

L(t)

I

www.ti.com

Board Features

The selection of resistors is not arbitrary, for matching and noise performance, the CSH current should be
set to be around 100 µA. This current does not flow in the LEDs and will not affect either off state LED
current or the regulated LED current. CSH current can be above or below this value, but high side
amplifier offset characteristics and jitter performance may be affected slightly.

5.3

Inductor Selection

The inductor should be chosen so that the current ripple (

Δ

i

L

) is between 20% and 40% of the average

current (<I

L

>) through the inductor.

Figure 3. Inductor Current Waveform

The worst case ripple is seen when the input voltage is at its lowest magnitude. This is true if we can say
that the output voltage stays relatively constant.

Design Example:

V

O

28V

V

IN-MIN

= 18V

V

IN-NOM

= 24V

V

IN-MAX

= 36V

I

LED

= 1A

Buck Boost Convertion Ratio:

(5)

Therefore:

(6)

D @ V

IN-MAX

= 0.436

D @ V

IN-MIN

= 0.609

(7)

t

ON

= @ V

IN-MAX

= 0.742 µs

t

ON

= @ V

IN-MIN

= 1.05 µs

Calculate average input current: The average input current is equal to the average inductor current.

(8)

Assume efficiency = 85%

(9)

I

IN

= 915 mA @ V

IN

= 36V

5

SNVA376A – December 2008 – Revised May 2013

AN-1907 LM3423 Buck-Boost Configuration Evaluation Board

Submit Documentation Feedback

Copyright © 2008–2013, Texas Instruments Incorporated

Содержание AN-1907 LM3423

Страница 1: ...ation while providing inherent input voltage feed forward compensation The LM3423 includes a high voltage start up regulator that operates over a wide input range of 4 5V to 75V The PWM controller is designed for high speed capability including a switching frequency range to 2 0 MHz Additional features include zero current shutdown error amplifier precision reference logic compatible DIM input sui...

Страница 2: ...the use of on board jumpers refer to Table 2 Table 2 LM3423 Evaluation Board Jumpers Jumper Designation Function or Use Notes J1 Enable EN OPEN Disables LM3423 CLOSED Enables LM3423 J2 Current Limit IS OPEN Disables MOSFET RDS ON current sensing Q5 CLOSED Enables MOSFET RDS ON current sensing Q5 J3 Current Limit IS OPEN Disables external sense resistor MOSFET current sensing Q5 CLOSED Enables exte...

Страница 3: ...Top View LM3423 Pin Connection 5 Board Features This evaluation board has all the necessary connections and jumpers to evaluate the LM3423 controller in a boost converter topology with the following operating features and options 5 1 Setting Average LED Current The LM3423 uses peak current mode control to regulate the boosted output voltage An external current sense resistor RSENSE i e R9 in serie...

Страница 4: ... to understanding the relationship between VCSH and VSNS because VSNS and RSNS set the LED current The high side amplifier in forces its input terminals to equal potential Because of this the VSNS voltage is forced across RHSP Another way to view this is that the amplifier s output transistor pulls current through R7 RHSP until VHSP VHSN and this happens when the voltage across R7 is equal to VSNS...

Страница 5: ...n so that the current ripple ΔiL is between 20 and 40 of the average current IL through the inductor Figure 3 Inductor Current Waveform The worst case ripple is seen when the input voltage is at its lowest magnitude This is true if we can say that the output voltage stays relatively constant Design Example VO 28V VIN MIN 18V VIN NOM 24V VIN MAX 36V ILED 1A Buck Boost Convertion Ratio 5 Therefore 6...

Страница 6: ...cle Two methods of switch current sensing are available on the evaluation board The first is accomplished through the use of an external sense resistor which allows for higher accuracy in sensing the peak current For the LM3423 evaluation board the sense resistor R6 can be utilized using the jumper configuration as described in Table 4 Figure 4 External RSENSE ISW Current Sense Table 4 External RS...

Страница 7: ... with a pulse width modulated PWM signal By reducing the average LED current light from the LEDs is reduced This dimming method allows the converter to operate the LEDs at a specific peak output current level iL which is usually a set point determined by the LED manufacturer This allows the LED to illuminate with a consistent light color while still having the ability to reduce its lumens output T...

Страница 8: ... Fault Protection Flag The LM3423 can be configured with fault protection by using the fault flag indicator FLT pin 9 When a fault condition is detected the FLT pin will go high pulled up to VIN by resistor R2 5 8 Compensation The LM3423 s error amplifier EA is a transconductance type amplifier which allows for easy single pin compensation When a capacitor is used on the output of the converter to...

Страница 9: ...oltage protection OVP with programmable hysteresis feature is available on the LM3423 to protect the device from damage when the boosted output voltage goes above a maximum value The OVP threshold is set up by the resister divider network of R22 and R20 which is referenced to the regulated output voltage VO The OVP threshold and hysteresis can be programmed completely independent of each other OVP...

Страница 10: ...will therefore release when VO reaches 42V R20 562 kΩ R22 12 4 kΩ 5 11 Under Voltage Protection The LM3423 can be configured for under voltage lockout UVLO protection with hysteresis using the dimming input nDIM pin 8 and a resistor divider from input voltage to ground UVLO protects the power devices during power supply startup and shutdown to prevent operation at voltages less than the minimum op...

Страница 11: ...LO set point resistor divider When the UVLO pin exceeds 1 24V the current source is activated to instantly raise the voltage at the UVLO pin When the UVLO pin voltage falls below the 1 24V threshold the current source is turned off causing the voltage at the UVLO pin to fall The UVLO hysteresis range can be user adjusted using the gain resistor R26 Figure 10 UVLO Circuit During Normal Operation 11...

Страница 12: ... UVLO Circuit with R26 Populated for Small Hysteresis and Large Input Voltage 6 Evaluation Board Test Procedure Proper Board Connections Be sure to choose the correct wire size when connecting the source supply and load Monitor the current into and out of the unit under test UUT Monitor the voltages directly at the board terminals as resistive voltage drops along the wires may decrease measurement...

Страница 13: ...23 R3 Q7 Q4 VO R26 CONN1 LM3423 R29 C14 Q6 Z1 9 10 12 11 TIMR FLT LRDY DPOL VIN TP9 R2 TP11 C6 TP5 R15 J6 VIN J4B Q5 J4A R5 D2 J2 TP10 R20 OVP RPD VIN C18 R22 Q3 VO www ti com LM3432 Evaluation Board Schematic 7 LM3432 Evaluation Board Schematic Figure 12 LM3423 Evaluation Board All features and external components shown 13 SNVA376A December 2008 Revised May 2013 AN 1907 LM3423 Buck Boost Configur...

Страница 14: ...MOSFET TO 252 Vishay SUD40N10 25 E3 40A 100V Q3 SOT 23 200mA 40V Fairchild MMB3904 Q6 SOT 23 200mA 40V Fairchild MMB2907 Q7 Q9 N channel MOSFET SOT23 Fairchild 2N7002 200mA 60V R2 R3 R15 Resistor 0805 100 kΩ Vishay CRCW08051003F R5 0Ω R6 Resistor 2512 0 06Ω Vishay WSL2512R0600FEA R7 R8 Resistor 0805 1 kΩ Vishay CRCW08051001F R9 Resistor 1812 0 1 Panasonic ERJL12KF10CU R13 R31 R23 Resistor 0805 10k...

Страница 15: ... TP6 R31 VIN J5 VCC R23 R3 Q7 Q4 Q4 OVP RPD VIN C18 R22 Q3 VO VO R26 CONN1 www ti com LM3421 Buck Boost Design Example High Side Current Sense High Speed Dimming 9 LM3421 Buck Boost Design Example High Side Current Sense High Speed Dimming Figure 13 LM3421 Design Example High Side Current Sense with High Speed Dimming 15 SNVA376A December 2008 Revised May 2013 AN 1907 LM3423 Buck Boost Configurati...

Страница 16: ...1 R13 C5 L1 CONN2 C17 C1 R14 C8 VIN VIN PGND VIN TP8 R27 VO D1 C16 C7 C11 C4 R30 CONN1 LM3421 Buck Boost Design Example High Side Current Sense www ti com 10 LM3421 Buck Boost Design Example High Side Current Sense Figure 14 LM3421 Design Example High Side Current Sense 16 AN 1907 LM3423 Buck Boost Configuration Evaluation Board SNVA376A December 2008 Revised May 2013 Submit Documentation Feedback...

Страница 17: ...esponsible for compliance with all legal regulatory and safety related requirements concerning its products and any use of TI components in its applications notwithstanding any applications related information or support that may be provided by TI Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which anticipate dangerous consequences of failur...

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