TC1313 datasheet(22/32 Pages) MICROCHIP

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TC1313

DS21974B-page 22

5.8

Design Example

VOUT1 = 2.0V @ 500 mA

VOUT2 = 3.3V @ 300 mA

VIN =5V ±10%

L= 4.7 µH

Calculate PWM mode inductor ripple current

Nominal Duty

Cycle = 2.0V/5.0V = 40%

P-channel

Switch-on time = 0.40 x 1/(2 MHz) = 200 ns

VL =(VIN-VOUT1)=3V

ΔIL =(VL/L) x TON =128 mA

Peak inductor current:

IL(PK) =IOUT1+1/2ΔIL = 564 mA

Switcher power loss:

Use efficiency estimate for 1.8V from Figure 2-7

Efficiency = 84%, PDISS1 =190 mW

Resistor Divider:

RTOP =200 kΩ

RBOT =133 kΩ

LDO Output:

PDISS2 =(VIN(MAX) –

VOUT2(MIN))x IOUT2(MAX)

PDISS2 = (5.5V – (0.975) x 3.3V) x 300 mA

PDISS2 =684.8 mW

Total

Dissipation = 190 mW + 685 mW = 875 mW

Junction Temp Rise and Maximum Ambient

Operating Temperature Calculations

10-Pin MSOP

(4-Layer Board with internal Planes)

θJA =113° C/Watt

Junction Temp.

Rise = 875 mW x 113° C/Watt = 98.9°C

Max. Ambient

Temperature = 125°C - 98.9°C

Max. Ambient

Temperature = 26.1°C

10-Pin DFN

θJA = 41° C/Watt (4-Layer Board with

internal planes and 2 vias)

Junction Temp.

Rise = 875 mW x 41° C/Watt = 35.9°C

Max. Ambient

Temperature = 125°C - 35.9°C

Max. Ambient

Temperature = 89.1°C

This is above the +85°C max. ambient temperature.

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