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LT1977

1977f

APPLICATIO S I FOR ATIO

tr = (VIN/1.1)ns

tf = (VIN/1.8)ns

tIR = tIF = (IOUT/0.05)ns

f = switch frequency

Example: with VIN = 12V, VOUT = 5V and IOUT = 1A:

TBD

BOOST

()( ) ( )

()()()( )(

)

()

() + () =

−

03 1

57 6

1 2 1 12 500 3

0 04

0 125 0 172

0 297

51 32

0 002

12 0 0015

5 0 003

0 033

Total power dissipation is:

PTOT = 0.297 + 0.065 + 0.033 = 0.40W

Thermal resistance for the LT1977 package is influenced

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

plane under the FE16 package, thermal resistance will be

about 45°C/W. No plane will increase resistance to about

150°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 + QJA (PTOT)

With the FE16 package (QJA = 45°C/W) at an ambient

temperature of 70°C:

TJ = 70 + 45(0.40) = 98°C

Input Voltage vs Operating Frequency Considerations

The absolute maximum input supply voltage for the LT1977

is specified at 60V. 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 1A and fOSC = 500kHz may be easily achievable, si-

multaneously raising VIN to 60V and fOSC to 700kHz is not

possible. Nevertheless, input voltage transients up to 60V

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 control. A potential limitation

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 con-

tinuous 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 control problem arises if the LT1977 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,lowVOUTand

high fOSC can result in an unacceptably short minimum

switch on time. Cycle skipping and/or Burst Mode be-

havior will result causing an increase in output voltage

ripple while maintaining the correct output voltage.

FREQUENCY COMPENSATION

Before starting on the theoretical analysis of frequency

response the following should be remembered—the worse

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