LT1950IGN datasheet(13/20 Pages) LINER

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LT1950

sn1950 1950fs

Synchronizing

The SYNC pin is used to synchronize the LT1950 main

oscillator to an external clock. The SYNC pin can be driven

directly from a logic level output, requiring less

than 0.8V for a logic level low and greater than 2.2V for a

logic level high. Duty cycle must be between 10% and

90%. When synchronizing the part, slope compensation

will be reduced by approximately SYNC f/fOSC. If the

reduction of slope compensation affects performance,

RSLOPE can be reduced to increase slope compensation

and reestablish correct operation. If unused, the pin is left

open or shorted to ground. If left open, be aware that the

internal pin resistance is 20k and board layout should be

checked to avoid noise coupling to the pin.

SLOPE COMPENSATION

Programmability

The LT1950 allows its default level of slope compensation

to be easily increased by use of a single resistor connected

between the SLOPE pin and the VREF pin. The ability to

adjust slope compensation allows the designer to tailor his

application for a wider inductor value range as well as to

optimize the loop bandwidth. A resistor, RSLOPE, con-

nected between the SLOPE pin and VREF increases the

LT1950 slope compensation from its default level to as

high as 3X of default. The curves in Figure 7 show the

typical ISENSE maximum threshold vs duty cycle for vari-

ous values of RSLOPE. It can be seen that slope compensa-

tion subtracts from the maximum ISENSE threshold as duty

cycle increases from 0%. For example, with RSLOPE open,

ISENSE max threshold is 100mV at low duty cycle, but falls

to approximately 86mV at 80% duty cycle. This must be

accounted for when designing a converter to operate up to

a maximum load current and over a given duty cycle range.

The application and inductor value will define the

minimum amount of slope compensation. Refer to the

APPLICATIO S I FOR ATIO

Electrical Characteristics for 1X, 2X and 3X default slope

compensation vs RSLOPE.

Requirement in Current Mode Converters/Advantage

of Adjustability

The LT1950 uses a current mode architecture to provide

fast response to load transients and to ease frequency

compensation requirements. Current mode switching regu-

lators which operate with duty cycles above 50% and have

continuous inductor current, must add slope compensa-

tion to their current sensing loop to prevent subharmonic

oscillations. (For more information on slope compensa-

tion see Application Note 19). Typical current mode switch-

ing regulators have a fixed internal slope compensation.

This can place constraints on the value of the inductor. If

too large an inductor is used, the fixed internal slope

compensation will be greater than needed, causing opera-

tion to approach voltage mode. If too small an inductor is

used, the fixed internal slope compensation will be too

small, resulting in subharmonic oscillations. The LT1950

increases the range of usable inductor values by allowing

slope compensation to be adjusted externally.

Figure 7. ISENSE Maximum Threshold vs Duty Cycle

DUTY CYCLE (%)

1950 F07

100

RSLOPE = OPEN

RSLOPE = 8k

RSLOPE = 3.3k

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