LT1776CS8 datasheet(10/20 Pages) LINER

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LT1776

Input Voltage vs Operating Frequency Considerations

The absolute maximum input supply voltage for the LT1776

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 500mA and fOSC = 200kHz may be easily achievable,

simultaneously raising VIN to 60V and fOSC to 400kHz is

not possible. Nevertheless, input voltage

transients up to

60V can usually be accommodated, assuming the result-

ing increase in internal dissipation is of insufficient time

duration 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:

OUT

IN OSC

in t

()

where:

VIN = input voltage

VOUT = output voltage

VF = Schottky diode forward drop

fOSC = switching frequency

It is important to understand the nature of minimum

switch ON time as given in the data sheet. This test is

intended to mimic behavior under short-circuit condi-

tions. It is performed with the VC control voltage at its

clamp level (VCL) and uses a fixed resistive load from VSW

to ground for simplicity. The resulting ON time behavior is

overconservative as a general operating design value for

two reasons. First, actual power supply application cir-

cuits present an inductive load to the VSW node. The

APPLICATIONS INFORMATION

resulting ramping current behavior helps overdrive the

current comparator (current mode switching) and reduce

its propagation delay, hastening output switch turnoff.

Second, and more importantly, actual power supply op-

eration involves a feedback amplifier that adjusts the VC

node control voltage to maintain proper output voltage. As

progressively shorter ON times are required, the feedback

loop acts to reduce VC, and the resulting overdrive further

reduces the propagation delay in the current comparator.

A suggested worst-case limit for minimum switch ON time

in actual operation is 350ns.

A potential controllability problem arises if the LT1776 is

called upon to produce an ON time shorter than its ability.

Feedback loop action will lower then reduce the VC control

voltage to the point where some sort of cycle-skipping or

odd/even cycle 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

minimum switch ON time. Cycle skipping and/or odd/

even cycle behavior will result although correct output

voltage is usually maintained.

Minimum Load Considerations

As discussed previously, a lightly loaded LT1776 with VC

pin control voltage below the boost threshold will operate

in low dV/dt mode. This affords greater controllability at

light loads, as minimum tON requirements are relaxed.

However, some users may be indifferent to pulse skipping

behavior, but instead may be concerned with maintaining

maximum possible efficiency at light loads. This require-

ment can be satisfied by forcing the part into Burst Mode

operation. The use of an external comparator whose

Burst Mode is a trademark of Linear Technology Corporation.

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