LT1680ISW datasheet(9/16 Pages) LINER

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LT1680

OPERATION

Slope Compensation

For duty cycles greater than 50%, slope compensation is

required to prevent current mode duty cycle instability in

the regulator control loop. The LT1680 employs internal

slope compensation that is adequate for most applica-

tions. However, if additional slope compensation is de-

sired, it is available through the SL/ADJ pin. Excessive

slope compensation will cause reduction in maximum

load current capability and is generally not desirable.

APPLICATIONS INFORMATION

RSENSE Selection for Input Current Limit

RSENSE generates a voltage that is proportional to the

inductor current for use by the LT1680 current sense

amplifier. The value of RSENSE is based on the required

input current. The average current limit function has a

typical threshold of 120mV/RSENSE, or:

RSENSE = 120mV/ILIMIT

Operation with VSENSE common mode voltage below 4.5V

may slightly degrade current limit accuracy. See Average

Current Limit Threshold Tolerance vs Common Mode

Voltage in the Typical Performance Characteristics sec-

tion for more information.

Output Voltage Programming

Output voltage is programmed through a resistor feed-

back network to the VFB pin (Pin 7) on the LT1680. This pin

is the inverting input of the error amplifier, which is

internally referenced to 1.25V. The divider is ratioed to

provide 1.25V at the VFB pin when the output is at its

desired value. Output voltage is thus set following the

relation:

VOUT = 1.25V(1 + R2/R1)

when an external resistor divider is connected to the

output as shown in Figure 1.

If high value feedback resistors are used, the input bias

current of the VFB pin (1µA maximum) could cause a slight

increase in output voltage. A Thevenin resistance at the

VFB pin of <5k is recommended.

Oscillator Components RCT and CCT

The LT1680 oscillator creates a modified sawtooth at its

timing node (CT) with a slow charge, rapid discharge

characteristic. The discharge time (tDISCH) corresponds to

the minimum off time of the PWM controller. This limits

maximum duty cycle (DCMAX) to:

DCMAX = 1 – (tDISCH)(fO)

This relation corresponds to the minimum value of the

timing resistor (RCT), which can be determined according

to the following relation (RCT vs DCMAX graph appears in

the Typical Performance Characteristics section):

RCT(MIN) ≈ [(0.8)(10

–3)(1 – DCMAX)]–1

Values for RCT > 15k yield maximum duty cycles above

90%. Given a timing resistor value, the value of the timing

capacitor (CCT) can then be determined for desired oper-

ating frequency (fO) using the relation:

≈

() ( )







()+()





 ()

100 10

185

175

2 5 10

3 375

/–

.–

–

A plot of Operating Frequency vs RCT and CCT is shown in

Figure 2. Typical 100kHz operational values are CCT =

1000pF and RCT = 16.9k.

LT1680

VOUT

VFB

1680 F01

SGND

Figure 1. Programming LT1680 Output Voltage

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