LT1766_1 datasheet(20/30 Pages) LINER

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LT1766/LT1766-5

1766fc

APPLICATIONS INFORMATION

a >100MHz oscilloscope must be used, and waveforms

should be observed on the leads of the package. This

switch off spike will also cause the SW node to go below

ground. The LT1766 has special circuitry inside which

mitigates this problem, but negative voltages over 0.8V

lasting longer than 10ns should be avoided. Note that

100MHz oscilloscopes are barely fast enough to see the

details of the falling edge overshoot in Figure 7.

A second, much lower frequency ringing is seen during

switch off-time if load current is low enough to allow the

inductor current to fall to zero during part of the switch

off-time (see Figure 8). Switch and diode capacitance

resonate with the inductor to form damped ringing at 1MHz

to 10 MHz. This ringing is not harmful to the regulator

and it has not been shown to contribute signiﬁcantly to

EMI. Any attempt to damp it with a resistive snubber will

degrade efﬁciency.

THERMAL CALCULATIONS

Power dissipation in the LT1766 chip comes from four

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

rent, and input quiescent current. The following formulas

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:

SW OUT

OUT

EFF

OUT

() ( )

()( )( )

(/ )

Boost current loss:

BOOST

OUT

()

Quiescent current loss:

OUT

()+

()

0 0015

0 003

RSW = Switch resistance (≈ 0.3) hot

tEFF = Effective switch current/voltage overlap time

= (tr + tf + tIr + tIf)

tr = (VIN/1.2)ns

tf = (VIN/1.7)ns

tIr = tIf = (IOUT/0.05)ns

f = Switch frequency

Example: with VIN = 40V, VOUT = 5V and IOUT = 1A:

Figure 8. Discontinuous Mode Ringing

1μs/DIV

VIN = 40V

VOUT = 5V

L = 47μH

10V/DIV

SWITCH NODE

VOLTAGE

INDUCTOR

CURRENT

AT IOUT = 0.1A

0.2A/DIV

1766 F08

Figure 7. Switch Node Resonance

50ns/DIV

1766 F07

2V/DIV

SW RISE

SW FALL

BOOST

= ()( ) ( ) +

()()()( )()

= () () =

−

03 1

97 10

1 2 1 40 200 10

004 0 388

043

51 36

002

40 0 0015

5 0 003

0 08

•/

•

)

( .

)

Total power dissipation in the IC is given by:

PTOT = PSW + PBOOST + PQ

= 0.43W + 0.02W + 0.08W = 0.53W

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