National Semiconductor LM3421 Technical data

July 31, 2008

LM3421, LM3423 N-Channel Controllers for Constant Current LED Drivers

General Description

The LM3421/LM3423 devices are versatile high voltage LED driver controllers. With the capability to be configured in a Buck, Boost, Buck-Boost (Flyback), or SEPIC topology, and an input operating voltage rating of 75V, these controllers are ideal for illuminating LEDs in a very diverse, large family of applications.

Adjustable high-side current sense with a typical sense voltage of 100mV allows for tight regulation of the LED current with the highest efficiency possible. Output LED current regulation is based on peak current-mode control with predictive Off-Time Control. This method of control eases the design of loop compensation while providing inherent input voltage feed-forward compensation.

The LM3421/LM3423 include a high-voltage startup regulator that operates over a wide input range of 4.5V to 75V. The internal PWM controller is designed for adjustable switching frequencies of up to 2.0MHz, thus enabling compact solutions. Additional features include: “zero” current shutdown, precision reference, logic compatible DIM input suitable for fast PWM dimming, cycle-by-cycle current limit, and thermal shutdown.

The LM3423 also includes an LED output status flag, a fault flag, a programmable fault timer, and a logic input to select the polarity of the dimming output driver.

Features

VIN range from 4.5V to 75V

■

2% Internal reference voltage (1.235V)

■

Current sense voltage adjustable from 20mV

■

High-side current sensing

■

2Ω MOSFET gate driver

■

Dimming MOSFET gate driver

■

Input under-voltage protection

■

Over-voltage protection

■

Low shutdown current, IQ < 1µA

■

Fast (50kHz) PWM dimming

■

Cycle-by-cycle current limit

■

Programmable switching frequency

■

LED output status flag (LM3423 only)

■

Fault timer pin (LM3423 only)

■

TSSOP EP 16-lead package (LM3421)

■

TSSOP EP 20-lead package (LM3423)

■

Applications

LED Drivers

■

Constant-Current Buck-Boost Regulator

■

Constant-Current Boost Regulator

■

Constant-Current Flyback Regulator

■

Constant-Current SEPIC Regulator

■

Thermo-Electric Cooler (Peltier) Driver

■

Typical Application Circuit

Boost LED Driver

30067331

Connection Diagrams

Top View

LM3421, LM3423

16-Lead TSSOP EP

NS Package Number MXA16A

30067304

NS Package Number MXA20A

Ordering Information

Order Number Spec. Package Type NSC Package

Drawing

LM3421MH NOPB TSSOP-16 EP MXA16A 92 Units, Rail

LM3421MHX NOPB TSSOP-16 EP MXA16A 2500 Units, Tape and Reel

LM3423MH NOPB TSSOP-20 EP MXA20A 73 Units, Rail

LM3423MHX NOPB TSSOP-20 EP MXA20A 2500 Units, Tape and Reel

Top View

20-Lead TSSOP EP

Supplied As

30067366

Pin Descriptions

LM3423 LM3421 Name Function

1 1

2 2 EN

3 3 COMP

4 4 CSH

5 5 RCT

6 6 AGND

7 7 OVP

8 8 nDIM

9 - FLT

Power supply input (4.5V-75V). Bypass with 100nF capacitor to AGND as close to the device

as possible in the circuit board layout.

Enable: Pull to ground for zero current shutdown. Tie directly to VIN for automatic startup at

4.1V.

Compensation: PWM controller error amplifier compensation pin. This pin connects through a series resistor-capacitor network to AGND.

Current Sense High: Output of the high side sense amplifier and input to the main regulation loop error amplifier.

Resistor Capacitor Timing: External RC network sets the predictive “off-time” and thus the switching frequency. The RC network should be placed as close to the device as possible in the circuit board layout.

Analog Ground: The proper place to connect the compensation and timing capacitor returns. This pin should be connected via the circuit board to the PGND pin through the EP copper circuit board pad.

Over-Voltage Protection sense input: 1.24V threshold with hysteresis that is user programmable by the selection of the OVP Over-Voltage Lock-Out (OVLO) resistor divider network.

Not DIM input: Dual function pin. Primarily used as the Pulse Width Modulation (PWM) input. When driven with a resistor divider from VIN, this pin also functions as a user programmable VIN Under-Voltage Lock-Out (UVLO) with 1.24V threshold and programmable hysteresis by the UVLO resistor divider network. The PWM and UVLO functions can be performed simultaneously.

Fault flag: This is an N-channel MOSFET open drain output. The FLT pin transitions to the high (open) state once the Fault Timer has timed out and latched the controller off.

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LM3423 LM3421 Name Function

Fault Timer: The fault timer is comprised of an external capacitor, an internal charging current

10 - TIMR

source, an internal discharge N-channel MOSFET, and a comparator that latches the fault condition when the threshold voltage (1.24V) is exceeded.

11 - LRDY

12 - DPOL

LED Ready status flag: This is an N-channel MOSFET open drain output which pulls down whenever the LED current is not in regulation.

Dim Polarity: Selects the polarity of the DIM driver output. Tie to VCC or leave open for low side dimming, tie to ground for high side dimming.

13 9 DDRV External Dimming MOSFET Gate Drive: Gate driver output responding to nDIM input.

14 10 PGND

Power Ground: GATE and DDRV gate drive ground current return pin. This pin should be connected via the circuit board to the AGND pin through the EP copper circuit board pad.

15 11 GATE Main switching MOSFET gate drive output.

16 12

Internal Regulator Bypass: 6.9V low dropout linear regulator output. Bypass with a 2.2µF–

3.3µF, ceramic type capacitor to PGND.

Main Switch Current Sense input: This pin is used for current mode control and cycle-by-cycle

17 13 IS

current limit. This pin can be tied to the drain of the main N-channel MOSFET switch for R

sensing or tied to a sense resistor installed in the source of the same device.

DS(ON)

Resistor Pull-Down: This is an open drain N-channel MOSFET which is used to pull-down the

18 14 RPD

low side of all external resistor dividers (VIN UVLO, OVP). This pin allows for system “zerocurrent” shutdown.

High Side Sense Positive: LED current sense positive input. An external resistor sets a reference current flowing into this pin from the programmed high-side sense voltage. Although

19 15 HSP

the current into this pin can be set to values ranging from 10µA through 1mA, a value of 100µA is recommended. This pin is a virtual ground whose potential is set by the voltage on the HSN pin.

High Side Sense Negative: This pin sets the reference voltage for the HSP input. An external

20 16 HSN

resistor of the same value as that used on the HSP pin should be connected from this pin to the negative side of the current sense resistor.

EP (21) EP (17) EP

EP: Star ground, connecting AGND and PGND. For thermal considerations please refer to (Note 4) of the Electrical Characteristics table.

LM3421, LM3423

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Absolute Maximum Ratings (Notes 1, 2)

If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications.

VIN, EN, RPD, nDIM -0.3V to 76.0V

LM3421, LM3423

OVP, HSP, HSN, LRDY, FLT, DPOL

RCT -0.3V to 76.0V

-1mA to +5mA continuous

IS -0.3V to 76.0V

TIMR -0.3V to 7.0V

COMP, CSH -0.3V to 6.0V

GATE, DDRV -0.3V to V

-1mA continuous

-0.3V to 76.0V

-100µA continuous

-2V for 100ns

-1mA continuous

-0.3V to 8.0V

-100µA to +100µA

-200µA to +200µA

Continuous

PGND -0.3V to 0.3V

-2.5V to 2.5V for 100ns

Maximum Junction Temperature (Internally Limited)

Storage Temperature Range −65°C to +150°C Maximum Lead Temperature

(Soldering) (Note 5) Continuous Power Dissipation

Internally Limited

(Notes , 4) ESD Susceptibility

(Note 6) Human Body Model 2kV Machine Model 200V Charge Device Model 500V

Operating Conditions (Notes 1, 2)

Operating Junction Temperature Range (Note 7) −40°C to +150°C

Input Voltage V

-2.5V for 100ns

VCC+2.5V for 100ns

-1mA to +1mA continuous

Electrical Characteristics (Note 2)

Specifications in standard type face are for TJ = 25°C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40°C to +125°C). Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical

values represent the most likely parametric norm at TJ = +25°C, and are provided for reference purposes only. Unless otherwise stated the following condition applies: VIN = +14V.

Symbol Parameter Conditions

STARTUP REGULATOR

CCREG

CCLIM

VCC Regulation ICC = 0mA 6.30

VCC Current Limit VCC = 0V

Quiescent Current EN = 3.0V, Static

Shutdown Current EN = 0V 0.1 1.0

VCC SUPPLY

CCUV

VCC UVLO Threshold VCC Increasing

VCC Decreasing

CCHYS

VCC UVLO Hysteresis 0.1

EN THRESHOLDS

EN Startup Threshold EN Increasing 1.75 2.40

EN Decreasing 0.80 1.63

STHYS

EN Startup Hysteresis

EN Pulldown Resistance EN = 1V

CSH THRESHOLDS

CSH High Fault CSH Increasing 1.6

CSH Low Condition on LRDY

CSH increasing

Pin (LM3423 only)

OV THRESHOLDS

OVP

HYS

OVP OVLO Threshold OVP Increasing

OVP Hysteresis Source

OVP Active (high)

Current

DPOL THRESHOLDS

DPOL

DPOL Logic Threshold DPOL Increasing 2.0

THRESH

Min(Note 7) Typ(Note 8) Max(Note 7)

6.90 7.35 V

20 25

2 3

4.17 4.50

3.70 4.08

0.1

0.45 0.82 1.30

1.0

1.185 1.240 1.285 V

20 23 25 µA

2.3 2.6 V

165°C

300°C

4.5V to 75V

Units

µA

MΩ

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LM3421, LM3423

Symbol Parameter Conditions

DPOL

DPOL Pullup Resistance

Min(Note 7) Typ(Note 8) Max(Note 7)

500 1200

Units

KΩ

FAULT TIMER

FLT

FLTTH

Fault Threshold

Fault Pin Source Current

1.185 1.240 1.285 V

10 11.5 13 µA

ERROR AMPLIFIER

REF

Error Amplifier Input Bias

CSH Reference Voltage With Respect to AGND 1.210

Current

-0.6 0 0.6

1.235 1.260 V

µA COMP Sink / Source Current 22 30 35

Transconductance 100 µA/V

Linear Input Range (Note 9) ±125 mV

Transconductance Bandwidth

(Note 9)

-6dB Unloaded Response

0.5 1.0

MHz

OFF TIMER

Minimum Off-time

RCT

RCT Reset Pull-down Resistance

RCT

VIN/25 Reference Voltage VIN = 14V 540

f Continuous Conduction

Switching Frequency

RCT = 1V through 1kΩ

2.2nF > CT > 470pF

35 75

36 120

565 585 mV

25/(CTRT)

Ω

PWM COMPARATOR

COMP to PWM Offset 700 800 900 mV

CURRENT LIMIT (IS)

LIM

Leading Edge Blanking Time 115 210 325

Current Limit Threshold 200

Delay to Output

LIM

245 300 mV

35 75

HIGH SIDE TRANSCONDUCTANCE AMPLIFIER

Input Bias Current 11.5 µA

Transconductance 20 119 mA/V

Input Offset Current -1.5 0 1.5 µA

Input Offset Voltage -7 0 7 mV

Transconductance Bandwidth

(Note 9)

CSH

= 100µA

250 500 kHz

GATE DRIVER (GATE)

SRC(GATE)

SNK(GATE)

GATE Sourcing Resistance GATE = High

GATE Sinking Resistance GATE = Low

2.0 6.0

1.3 4.5

Ω

DIM DRIVER (DIM, DDRV)

nDIM

SRC(DDRV)

SNK(DDRV)

VTH

HYS

nDIM / UVLO Threshold 1.185

nDIM Hysteresis Current 20

DDRV Sourcing Resistance DDRV = High

DDRV Sinking Resistance DDRV = Low

1.240 1.285 V

23 25 µA

13.5 30.0

3.5 10.0

Ω

PULL-DOWN N-CHANNEL MOSFETS

RPD

FLT

LRDY

RPD Pull-down Resistance

FLT Pull-down Resistance

LRDY Pull-down Resistance

145 300

135 300

Ω

THERMAL SHUTDOWN

HYS

Thermal Shutdown Threshold

Thermal Shutdown Hysteresis

165

°C

THERMAL RESISTANCE

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Symbol Parameter Conditions

Junction to Ambient (Note 4) 16L TSSOP EP 37.4

Min(Note 7) Typ(Note 8) Max(Note 7)

20L TSSOP EP 34.0

Junction to Exposed Pad (EP) 16L TSSOP EP 2.3

20L TSSOP EP 2.3

LM3421, LM3423

Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended

to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: All voltages are with respect to the potential at the AGND pin, unless otherwise specified.

Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=165°C (typical) and disengages at

TJ=140°C (typical).

Note 4: Junction-to-ambient thermal resistance is highly board-layout dependent. The numbers listed in the table are given for an reference layout wherein the 16L TSSOP package has its EP pad populated with 9 vias and the 20L TSSOP has its EP pad populated with 12 vias. In applications where high maximum power dissipation exists, namely driving a large MOSFET at high switching frequency from a high input voltage, special care must be paid to thermal dissipation issues during board design. In high-power dissipation applications, the maximum ambient temperature may have to be derated. Maximum ambient temperature (T

) is dependent on the maximum operating junction temperature (T

MAX

), and the junction-to ambient thermal resistance of the package in the application (θJA), as given by the following equation: T

MAX

). In most applications there is little need for the full power dissipation capability of this advanced package. Under these circumstances, no vias would be

MAX

required and the thermal resistances would be 104 °C/W for the 16L TSSOP and 86.7 °C/W for the 20L TSSOP. It is possible to conservatively interpolate between the full via count thermal resistance and the no via count thermal resistance with a straight line to get a thermal resistance for any number of vias in between these two limits.

Note 5: Refer to National’s packaging website for more detailed information and mounting techniques. http://www.national.com/packaging/

Note 6: Human Body Model, applicable std. JESD22-A114-C. Machine Model, applicable std. JESD22-A115-A. Field Induced Charge Device Model, applicable

std. JESD22-C101-C.

Note 7: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

Note 8: Typical numbers are at 25°C and represent the most likely norm.

Note 9: These electrical parameters are guaranteed by design, and are not verified by test.

= 125°C), the maximum power dissipation of the device in the application (P

J-MAX-OP

A-MAX

= T

J-MAX-OP

Units

°C/W

– (θJA × P

A-

D-

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Typical Performance Characteristics Taken from the standard evaluation board

LM3421, LM3423

Boost Efficiency vs. Input Voltage

(6 White LEDs, ~20V at 1A)

Boost Efficiency vs. Input Voltage

(11 White LEDs, ~46V at 1A)

Buck/Boost Efficiency vs. Input Voltage

(6 White LEDs, ~20V at 1A)

30067322

30067323

Average LED Current vs. PWM DIM Duty Cycle

(VIN = 12V, 6 White LEDs, ~20V at 1A)

30067321

LED Current vs. Ambient Temperature

(VIN = 12V, Nominal LED Current Set at 1A)

30067319

30067318

LED Current vs. Input Voltage

(Boost, 6 White LEDs, ~20V at 1A)

30067324

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