LM3421 datasheet(7/65 Pages) TI1

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LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

Electrical Characteristics

(1) (continued)

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 +150°C for LM3421Q0/LM3423Q0, TJ = −40°C to +125°C for all others). Specifications

that differ between the two operating ranges will be identified in the Temp Range column as Q0 for TJ = −40°C to +150°C

and as Q1 for TJ = −40°C to +125°C. If no temperature range is indicated then the specification holds for both Q1 and Q0.

Minimum and Maximum limits are specified 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.

Temp

Symbol

Parameter

Conditions

Min(2)

Typ(3)

Max(2)

Units

Range

PULL-DOWN N-CHANNEL MosFETS

RRPD

RPD Pull-down Resistance

300

145

350

RFLT

FLT Pull-down Resistance

300

145

Ω

350

RLRDY

LRDY Pull-down Resistance

300

135

350

THERMAL SHUTDOWN

TSD

Thermal Shutdown Threshold

(4)

165

210

°C

THYS

Thermal Shutdown Hysteresis

(4)

THERMAL RESISTANCE

θJA

Junction to Ambient (5)

16L TSSOP

37.4

°C/W

20L TSSOP

34.0

θJC

Junction to Exposed Pad (EP)

16L TSSOP

2.3

°C/W

20L TSSOP

2.3

(5)

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 (TA-MAX) is dependent on the maximum

operating junction temperature (TJ-MAX-OP = 125°C for Q1, or 150°C for Q0), the maximum power dissipation of the device in the

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

equation: TA-MAX = TJ-MAX-OP – (θJA × PD-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 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.

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