LTC7103 datasheet(20/48 Pages) AD

20 / 48 page

LTC7871

Rev. 0

For more information www.analog.com

The Typical Application on the first page of this data sheet

is a basic LTC7871 application circuit. In general, external

component selection is driven by the load requirements,

and begins with the DCR or RSENSE and inductor value.

Next, power MOSFETs are selected. Finally, VHIGH and

VLOW capacitors are selected.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant fre-

quency architectures by preventing subharmonic oscilla-

tions at high duty cycles. It is accomplished internally by

adding a compensating ramp to the inductor current sig-

nal at duty cycles in excess of 40%. Normally, this results

in a reduction of maximum inductor peak current for duty

cycles > 40%. However, the LTC7871 uses a scheme that

counteracts this compensating ramp, which allows the

maximum inductor peak current to remain unaffected

throughout all duty cycles.

Current Limit Programming

The ILIM pin is a 5-level logic input which sets the maxi-

mum current limit of the controller. Table 5 shows the five

ILIM settings. Please note that these settings represent

the peak inductor current setting. Because of the inductor

ripple current, the average output current is lower than

the peak current. Setting ILIM using a resistor divider

from V5 to SGND will allow the maximum current sense

threshold setting to not change when the 5V LDO is in

dropout at start-up. Please note that the ILIM pin has an

internal 200k pull down resistor to SGND and a 200k pull

up resistor to V5.

Table 5. ILIM Settings

ILIM Pin Voltage

Maximum Current Sense Threshold

DCR Sensing

RSENSE

10mV

12.5mV

1/4 VV5

20mV

25mV

Float

30mV

37.5mV

3/4 VV5

40mV

50mV

VV5

50mV

62.5mV

APPLICATIONS INFORMATION

SNSD+, SNSA+ and SNS– Pins

The SNSA+ and SNS– pins are the inputs to the current

comparators, while the SNSD+ and SNS– pins are the input

of an internal DC amplifier. The operating input voltage

range is 0V to 60V for all three sense pins. All the positive

sense pins that are connected to the current comparator

or the amplifier are high impedance with input bias cur-

rents of less than 1μA. The SNS– pin is not a high imped-

ance pin. For VLOW voltages greater than V5, the current

comparators derive their bias currents directly from the

SNS– pins. The SNS– pins should be connected directly

to VLOW. Care must be taken not to float these pins during

normal operation. Filter components, especially capaci-

tors, must be placed close to the LTC7871, and the sense

lines should run close together to a Kelvin connection

underneath the current sense element (Figure 2). Because

the LTC7871 is designed to be used with a very low value

sensing element to sense inductor current, without proper

care, the parasitic resistance, capacitance and inductance

will degrade the current sense signal integrity, making

the programmed current limit unpredictable. As shown in

Figure 3, resistor R1 is placed close to the output induc-

tor and capacitors C1 and C2 are close to the IC pins to

prevent noise coupling to the sense signal.

Figure 2.

COUT

TO SENSE FILTER LOCATED

NEXT TO THE CONTROLLER

7871 F02

Sense Lines Placement with Sense Resistor

Inductor DCR Sensing

The LTC7871 is specifically designed for high load current

applications requiring the highest possible efficiency; it is

capable of sensing the signal of an inductor DCR in the

milliohm range (Figure 3). The DCR is the DC winding

resistance of the inductor’s copper, which is often several

mΩ for high current inductors. In high current applica-

tions, the conduction loss of a high DCR or a sense resis-

tor will cause a significant reduction in power efficiency.

The SNSA+ pin connects to the filter that has a R1 • C1

time constant one-fifth of the L/DCR of the inductor. The

SNSD+ pin is connected to the second filter with the time

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