LTC3245 datasheet(13/18 Pages) LINER

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LTC3245

3245fa

For more information www.linear.com/LTC3245

applicaTions inForMaTion

ratherthandiscussingthespecifiedcapacitancevalue.For

example, over rated voltage and temperature conditions,

a 4.7μF, 10V, Y5V ceramic capacitor in an 0805 case may

not provide any more capacitance than a 1μF, 10V, X5R

or X7R available in the same 0805 case. In fact, over bias

and temperature range, the 1μF, 10V, X5R or X7R will

provide more capacitance than the 4.7μF, 10V, Y5V. The

capacitor manufacturer’s data sheet should be consulted

to determine what value of capacitor is needed to ensure

minimum capacitance values are met over operating

temperature and bias voltage. Below is a list of ceramic

capacitor manufacturers and how to contact them:

MANUFACTURER

WEBSITE

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

TDK

www.tdk.com

Layout Considerations

Due to the high switching frequency and transient cur-

rents produced by the LTC3245, careful board layout is

necessary for optimal performance. A true ground plane

and short connections to all capacitors will optimize

performance, reduce noise and ensure proper regulation

over all conditions.

When using the LTC3245 with an external resistor divider

it is important to minimize any stray capacitance to the

ADJ (OUTS/ADJ pin) node. Stray capacitance from ADJ

to C+ or C– can degrade performance significantly and

should be minimized and/or shielded if necessary.

Thermal Management

TheonchippowerdissipationintheLTC3245willcausethe

junction to ambient temperature to rise at rate of 40°C/W

or more. To reduce the maximum junction temperature, a

goodthermalconnectiontothePCboardisrecommended.

Connecting the die paddle (Pin 13) with multiple vias to a

largegroundplaneunderthedevicecanreducethethermal

resistanceofthepackageandPCboardconsiderably.Poor

board layout and failure to connect the die paddle (Pin 13)

to a large ground plane can result in thermal junction to

ambient impedance well in excess of 40°C/W.

Because of the wide input operating range it is possible to

exceed the specified operating junction temperature and

even reach thermal shutdown. Figure 3 shows the avail-

able output current vs temperature to ensure the 150°C

operating junction temperature is not exceed for input

voltages less than 20V.

Figure 3 assumes worst-case operating conditions. Under

someoperatingconditionsthepartcansupplymorecurrent

than shown without exceeding the 150°C operating junc-

tion temperature. When operating outside the constraints

of Figure 3 it is the responsibility of the user to calculate

worst-caseoperatingconditions(temperatureandpower)

to make sure the LTC3245’s specified operating junction

temperature is not exceeded for extended periods of time.

The 2:1 Step-Down, 1:1 Step-Down, and 1:2 Step-Up

Charge Pump Operation sections provide equations for

calculating power dissipation (PD) in each mode.

For example, if it is determined that the maximum power

dissipation (PD) is 1.2W under normal operation, then the

junction to ambient temperature rise will be:

Junction to ambient = 1.2W • 40°C/W = 48°C

Thus,theambienttemperatureunderthisconditioncannot

exceed102°Cifthejunctiontemperatureistoremainbelow

150°Candiftheambienttemperatureexceedsabout127°C

the device will cycle in and out of the thermal shutdown.

Figure 3. Available Output Current vs Temperature

TEMPERATURE (°C)

150

100

250

200

300

3245 F03

100

110

130

120

150

140

VIN < 20V

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