LTC3613 datasheet(24/44 Pages) AD

24 / 44 page

LTC7131-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge stor-

age requirement. During a load step, the output capac-

itor must instantaneously supply the current to support

the load until the feedback loop raises the switch current

enough to support the load. The time required for the

feedback loop to respond is dependent on the compensa-

tion and the output capacitor size. Typically, 3 to 4 cycles

are required to respond to a load step, but only in the first

cycle does the output drop linearly. The output droop,

VDROOP, is usually about 2 to 3 times the linear drop of

the first cycle. Thus, a good place to start with the output

capacitor value is approximately given by Equation 10.

COUT ≈ 2.5

ΔIOUT

fO • VDROOP

(10)

More capacitance may be required depending on the duty

cycle and load step requirements.

In most applications, the input capacitor is merely required

to supply high frequency bypassing, since the impedance

to the supply is very low. A 22µF ceramic capacitor is

usually enough for these conditions. Place this input

capacitor as close to the VIN pins as possible.

Thermal Considerations

In some applications where the LTC7131-1 is operated

at high ambient temperature, high VIN, high switching

frequency and maximum output current load, the heat

dissipated may exceed the maximum junction tempera-

ture of the part.

To avoid the LTC7131-1 from exceeding the maximum

junction temperature, current rating shall be derated in

accordance to Ambient Temperature vs Maximum Load

Current in the Typical Performance Characteristics.

The junction to ambient thermal resistance will vary

depending on the size amount of heat sinking copper

on the PCB board where the part is mounted, as well

as the amount of air flow on the device. Figure 11 and

Figure 12 show temperature derating with both heat sink

and airflow. Use the READ_TEMPERATURE_1 command

to check the die temperature at worst-case operating con-

ditions as a final check.

Figure 11. Temperature Derating Curve Based on

the DC2824A Demo Board

NO HEAT SINK

VIN = 12V

VOUT = 1.8V

fSW = 500kHz

DC2824A DEMOBOARD

0LFM

200LFM

400LFM

AMBIENT TEMPERATURE (°C)

100

125

150

on the DC2824A Demo Board

7131-1 F11

Figure 12. Temperature Derating Curve Based on

the DC2824A Demo Board

WITH HEAT SINK

VIN = 12V

VOUT = 1.8V

fSW = 500kHz

DC2824A DEMOBOARD

0LFM

200LFM

400LFM

AMBIENT TEMPERATURE (°C)

100

125

150

on the DC2824A Demo Board

7131-1 F12

Table 4 and Table 5 provide heat sink and thermal con-

ductive adhesive tape information.

Table 4. Heat Sink Manufacturer (Thermally Conductive

Adhesive Tape Pre-Attached)

HEAT SINK

MANUFACTURER

PART NUMBER

WEBSITE

Cool Innovations

03-0202035U

www.coolinnovations.com

Table 5. Thermally Conductive Adhesive Tape Vendor

THERMALLY CONDUCTIVE

ADHESIVE TAPE

MANUFACTURER

PART NUMBER WEBSITE

Chomerics

T411

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