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LT3437

3437f

APPLICATIO S I FOR ATIO

(tr + tf + tIR + tIF)

tr = (VIN/0.6)ns

tf = (VIN/2)ns

tIR = tIF = (IOUT/0.05)ns

f = switch frequency

Example: with VIN = 40V, VOUT = 5V and IOUT = 250mA:

BOOST

= ( )(

)()

()

()()( )(

)

= () () =

()+ () =

1 0 25

92 12 0 25 40 200 3

0 008 0 092

0 1

50 25 30

0 005

40 0 0005

5 0 0008

0 024

Total power dissipation is:

PTOT = 0.1 + 0.065 + 0.024 = 0.13W

Thermal resistance for the LT3437 package is influenced

by the presence of internal or backside planes. With a full

plane under the package, thermal resistance will be about

°C for the FE and DD packages. No plane will increase

resistance to about 150

°C/W.Tocalculatedietemperature,

use the proper thermal resistance number for the desired

package and add in worst-case ambient temperature:

TJ = TA + QJA (PTOT)

With the DD package (QJA = 45°C/W) at an ambient

temperature of 70

°C:

TJ = 70 + 45(0.1) = 74.5°C

Input Voltage vs Operating Frequency Considerations

The absolute maximum input supply voltage for the LT3437

is specified at 80V. This is based solely on internal semi-

conductor junction breakdown effects. Due to internal

power dissipation, the actual maximum VIN achievable in

a particular application may be less than this.

A detailed theoretical basis for estimating internal power

loss is given in the section Thermal Considerations. Note

that AC switching loss is proportional to both operating

frequency and output current. The majority of AC switch-

ing loss is also proportional to the square of input voltage.

For example, while the combination of VIN = 40V, VOUT =

5V at 700mA and fOSC = 200kHz may be easily achievable,

simultaneously raising VIN to 80V and fOSC to 700kHz is not

possible. Nevertheless, input voltage transients up to 80V

can usually be accommodated, assuming the resulting

increase in internal dissipation is of insufficient time dura-

tion to raise die temperature significantly.

A second consideration is controllability. A potential limi-

tation occurs with a high step-down ratio of VIN to VOUT,

as this requires a correspondingly narrow minimum switch

on time. An approximate expression for this (assuming

continuous mode operation) is given as follows:

tON(MIN) = (VOUT + VF)/VIN(fOSC)

where:

VIN = input voltage

VOUT = output voltage

VF = Schottky diode forward drop

fOSC = switching frequency

A potential controllability problem arises if the LT3437 is

called upon to produce an on time shorter than it is able to

produce. Feedback loop action will lower, then reduce, the

VC control voltage to the point where some sort of cycle-

skipping or Burst Mode behavior is exhibited.

In summary:

1. Be aware that the simultaneous requirements of high

VIN, high IOUT and high fOSC may not be achievable in

practice due to internal dissipation. The Thermal Con-

siderations section offers a basis to estimate internal

power. In questionable cases, a prototype supply should

be built and exercised to verify acceptable operation.

2. The simultaneous requirements of high VIN, low VOUT

and high fOSC can result in an unacceptably short mini-

mum switch on time. Cycle skipping and/or Burst Mode

behavior will result causing an increase in output volt-

age ripple while maintaining the correct output voltage.

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