LT1776CS8 datasheet(8/20 Pages) LINER

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LT1776

OPERATIO

The system as previously described handles heavy loads

(continuous mode) at good efficiency, but it is actually

counterproductive for light loads. The method of jam-

ming charge into the PNP bases makes it difficult to turn

them off rapidly and achieve the very short switch ON

times required by light loads in discontinuous mode.

Furthermore, the high leading edge dV/dt rate similarly

adversely affects light load controllability.

The solution is to employ a “boost comparator” whose

inputs are the VC control voltage and a fixed internal

threshold reference, VTH. (Remember that in a current

mode switching topology, the VC voltage determines the

peak switch current.) When the VC signal is above VTH, the

previously described “high dV/dt” action is performed.

When the VC signal is below VTH, the boost pulses are

absent, as can be seen in the Low dV/dt Mode Timing

Diagram. Now the DC current, activated by the SWON

signal alone, drives Q4 and this transistor drives Q1 by

itself. The absence of a boost pulse, plus the lack of a

second NPN driver, result in a much lower slew rate which

aids light load controllability.

A further aid to overall efficiency is provided by the

specialized bias regulator circuit, which has a pair of

inputs, VIN and VCC. The VCC pin is normally connected to

the switching supply output. During start-up conditions,

the LT1776 powers itself directly from VIN. However, after

the switching supply output voltage reaches about 2.9V,

the bias regulator uses this supply as its input. Previous

generation buck controller ICs without this provision

typically required hundreds of milliwatts of quiescent

power when operating at high input voltage. This both

degraded efficiency and limited available output current

due to internal heating.

APPLICATIONS INFORMATION

Selecting a Power Inductor

There are several parameters to consider when selecting

a power inductor. These include inductance value, peak

current rating (to avoid core saturation), DC resistance,

construction type, physical size, and of course, cost.

In a typical application, proper inductance value is dictated

by matching the discontinuous/continuous crossover point

with the LT1776 internal low-to-high dV/dt threshold. This

is the best compromise between maintaining control with

light loads while maintaining good efficiency with heavy

loads. The fixed internal dV/dt threshold has a nominal

value of 1.4V, which referred to the VC pin threshold and

control voltage to switch transconductance, corresponds

to a peak current of about 200mA. Standard buck con-

verter theory yields the following expression for induc-

tance at the discontinuous/continuous crossover:

OUT

















•

–

For example, substituting 40V, 5V, 200mA and 200kHz

respectively for VIN, VOUT, IPK and f yields a value of about

100

µH. Note that the left half of this expression is indepen-

dent of input voltage while the right half is only a weak

function of VIN when VIN is much greater than VOUT. This

means that a single inductor value will work well over a

range of “high” input voltage. And although a progres-

sively smaller inductor is suggested as VIN begins to

approach VOUT, note that the much higher ON duty cycles

under these conditions are much more forgiving with

respect to controllability and efficiency issues. Therefore

when a wide input voltage range must be accommodated,

say 10V to 40V for 5VOUT, the user should choose an

inductance value based on the maximum input voltage.

Once the inductance value is decided, inductor peak

current rating and resistance need to be considered. Here,

the inductor peak current rating refers to the onset of

saturation in the core material, although manufacturers

sometimes specify a “peak current rating” which is de-

rived from a worst-case combination of core saturation

and self-heating effects. Inductor winding resistance alone

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