Datasheet

TC1303A/TC1303B/TC1303C/TC1304
DS21949C-page 28 © 2008 Microchip Technology Inc.
5.8 Design Example
V
OUT1
= 2.0V @ 500 mA
V
OUT2
= 3.3V @ 300 mA
V
IN
= 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
V
L
=(V
IN
-V
OUT1
)=3V
∆I
L
=(V
L
/L) x T
ON
=128mA
Peak inductor current:
I
L(PK)
=I
OUT1
+1/2∆I
L
= 564 mA
Switcher power loss:
Use efficiency estimate for 1.8V from Figure 2-8
Efficiency = 84%, P
DISS1
=190mW
Resistor Divider:
R
TOP
=200kΩ
R
BOT
=133kΩ
LDO Output:
P
DISS2
=(V
IN(MAX)
–
V
OUT2(MIN)
)xI
OUT2(MAX)
P
DISS2
= (5.5V – (0.975) x 3.3V) x 300 mA
P
DISS2
=684.8mW
Tota l
Dissipation = 190 mW + 685 mW = 874 mW
Junction Temp Rise and Maximum Ambient
Operating Temperature Calculations
10-Pin MSOP (4-Layer Board with internal Planes)
Rθ
JA
=113°C/Watt
Junction Temp.
Rise = 874 mW x 113° C/Watt = 98.8°C
Max. Ambient
Temperature = 125°C - 98.8°C
Max. Ambient
Temperature = 26.3°C
10-Pin DFN
Rθ
JA
= 41° C/Watt (4-Layer Board with
internal planes and 2 vias)
Junction Temp.
Rise = 874 mW x 41° C/Watt = 35.8°C
Max. Ambient
Temperature = 125°C - 35.8°C
Max. Ambient
Temperature = 89.2°C
This is above the +85°C max. ambient temperature.