LT1339C datasheet(10/20 Pages) LINER

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LT1339

APPLICATIONS INFORMATION

RSENSE Selection for Output Current

RSENSE generates a voltage that is proportional to the

inductor current for use by the LT1339 current sense

amplifier. The value of RSENSE is based on the required

load 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 curve in the Typical Performance Characteristics

section for more information.

Output Voltage Programming

Output voltage is programmed through a resistor feed-

back network to VFB (Pin 9) on the LT1339. 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.

The output voltage is thus set following the relation:

VOUT = 1.25(1 + R2/R1)

when an external resistor divider is connected to the

output as shown in Figure 1.

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.

VOUT

1339 • F01

VFB

SGND

LT1339

Figure 1. Programming LT1339 Output Voltage

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 LT1339 oscillator creates a modified sawtooth wave

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

characteristic. The rapid discharge time corresponds to

Figure 2. Oscillator Frequency vs RCT, CCT

Average Current Limit

The average current limit function is implemented using

an external capacitor (CAVG) connected from IAVG to SGND

that forms a single pole integrator with the 50k

Ω output

TIMING RESISTOR (k

Ω)

LT1339 • F02

510

160

140

120

100

CCT = 3.3nF

CCT = 2.2nF

CCT = 1.5nF

CCT = 1.0nF

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