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LT1374

1374fb

APPLICATIONS INFORMATION

THERMAL CALCULATIONS

Power dissipation in the LT1374 chip comes from four

sources: switch DC loss, switch AC loss, boost circuit

current, and input quiescent current. The following formu-

las show how to calculate each of these losses. These

formulas assume continuous mode operation, so they

should not be used for calculating efficiency at light load

currents.

Switch loss:

ns I

SW OUT

OUT

() ( )

()( )( )

Boost current loss:

BOOST

OUT

()

Quiescent current loss:

OUT

()+ ()+







()

0 001

0 005

0 002

RSW = Switch resistance (≈ 0.07)

24ns = Equivalent switch current/voltage overlap time

f = Switch frequency

Example: with VIN = 10V, VOUT = 5V and IOUT = 3A:

BOOST

= (

)() ()

+ 



 ()( )









= () (

)

()+ ()+() ( ) =

−

007 3

24 10

3 10 500 10

0 32

0 36

0 68

53 50

015

10 0 001

5 0 005

0 002

004

••

Total power dissipation is 0.68 + 0.15 + 0.04 = 0.87W.

Thermal resistance for LT1374 package is influenced by

the presence of internal or backside planes. With a full

plane under the 16-lead TSSOP package, thermal resis-

tance will be about 40

°C/W. To calculate die temperature,

use the proper thermal resistance number for the desired

package and add in worst-case ambient temperature:

TJ = TA + θJA (PTOT)

With the TSSOP16 package (

θJA = 40°C/W), at an ambient

temperature of 50

°C,

TJ = 50 + 40 (0.87) = 85°C

For the DD package with a good copper plane under the

device, thermal resistance will be about 30

°C/W. For the

conditions above:

TJ = 50 + 30 (0.87) = 76°C

Die temperature is highest at low input voltage, so use

lowest continuous input operating voltage for thermal

calculations.

FREQUENCY COMPENSATION

Loop frequency compensation of switching regulators

can be a rather complicated problem because the reactive

components used to achieve high efficiency also intro-

duce multiple poles into the feedback loop. The inductor

and output capacitor on a conventional step-down con-

verter actually form a resonant tank circuit that can exhibit

peaking and a rapid 180

° phase shift at the resonant

frequency. By contrast, the LT1374 uses a “current mode”

architecture to help alleviate phase shift created by the

inductor. The basic connections are shown in Figure 9.

Figure 10 shows a Bode plot of the phase and gain of the

power section of the LT1374, measured from the VC pin to

the output. Gain is set by the 5.3A/V transconductance of

the LT1374 power section and the effective complex

impedance from output to ground. Gain rolls off smoothly

above the 600Hz pole frequency set by the 100

µF output

capacitor. Phase drop is limited to about 70

°. Phase

recovers and gain levels off at the zero frequency (

≈16kHz)

set by capacitor ESR (0.1

Ω).

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