LM2735XMFSLASHNOPB datasheet(44/59 Pages) TI

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RDSON

¸¸

¨¨

NFET

COND

Isw-rms = IIND D

1 +

IIND

PIND = IIN

2 x R

IND-DCR

( )

LM2735, LM2735-Q1

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

www.ti.com

Thermal Considerations (continued)

The LM2735 conduction loss is mainly associated with the internal NFET:

PCOND-NFET = I

SW-rms × RDSON × D

(40)

Figure 49. LM2735 Switch Current

(small ripple approximation)

(41)

PCOND-NFET = IIN

2 × R

DSON × D

(42)

(43)

The value for should be equal to the resistance at the junction temperature you wish to analyze. As an example,

at 125°C and VIN = 5 V, RDSON = 250 mΩ (see Typical Characteristics for value).

Switching losses are also associated with the internal NMOS switch. They occur during the switch on and off

transition periods, where voltages and currents overlap resulting in power loss.

The simplest means to determine this loss is to empirically measuring the rise and fall times (10% to 90%) of the

switch at the switch node:

PSWR = 1/2(VOUT × IIN × FSW × TRISE)

(44)

PSWF = 1/2(VOUT × IIN × FSW × TFALL)

(45)

PSW = PSWR + PSWF

(46)

Table 2. Typical Switch-Node Rise and Fall Times

VIN

VOUT

TRISE

TFALL

3 V

5 V

6 nS

4 nS

5 V

12 V

6 nS

5 nS

3 V

12 V

7 nS

5 nS

5 V

18 V

7 nS

5 nS

Quiescent Power Losses:

IQ is the quiescent operating current, and is typically around 4 mA.

PQ = IQ x VIN

(47)

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