TC1313-AM1EMF View Datasheet(PDF) - Microchip Technology

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TC1313-AM1EMF

500 mA Synchronous Buck Regulator,+ 300 mA LDO

Microchip Technology 

5.8 Design Example

VOUT1 = 2.0V @ 500 mA

VOUT2 = 3.3V @ 300 mA

VIN = 5V ±10%

L = 4.7µH

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)

RθJA = 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

RθJA = 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.

© 2005 Microchip Technology Inc.

TC1313

DS21974A-page 19

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