TSM108

10/13

Ω,

Ciss = 1nF.

Ω

The efficiency (

η) of a regulator is defined as the

ratio of the charging power (Pout) to the total pow-

er from the supply (Pin).

η = Pout/Pin

The output power is:

Pout=Iout x Vout where Iout is the charging cur-

rent (Vsense/Rsense = 625mA at full load) and

Vout is the regulated voltage (Vref(1+R1/R2) =

6V).

Pout = 3.75W

The input power can be found by adding the out-

put power (Pout) to the total power loss in the cir-

cuit (Plosses) i.e.

The power is lost partly on the chip and partly on

the external components which are mainly the di-

ode, the switch and the shunt. Plosses = Pchip +

Pswitch + Pdiode + Pshunt.

In Plosses, we neglect the losses in the inductor

(because the current through the inductor is

smoothened making the serial resistor of the in-

ductor very low), and the losses in the Gate

(charge and discharge).

a. The power lost in the chip is Pchip = Vcc x Icc.

(Vcc = 12V, Icc = 6mA) Pchip = 72m

Ω

b. The power lost in the switch depends on the ON

resistance of the switch and the current passing

through it. Also there is power loss in the switch

during switching time (commutation losses) and

that depends on the switching frequency and the

rise and fall time of the switching signal.

Rise time (Pchannel goes off) depends on the out-

put source current of the TSM108 and the input

gate capacitance of the Mosfet.

Trise = Ciss x Vgate / Isource

Fall time (Pchannel goes on) depends on the out-

put sink current of the TSM108 and the input gate

capacitance of the Mosfet .

Tfall = Ciss x Vgate / Isink

Trise = 150ns and Tfall = 300ns (Vgate is approx

12V).

where:

Prise = Iout x (Vcc+Vf) x Trise x PWMfreq / 2

Prise = 625mA x 12.7 x 150ns x 100kHz / 2.

Prise = 59.5mW

where:

Pfall = Iout x (Vcc+Vf) x Tfall x PWMfreq / 2

Pfall = 625mA x 12.7 x 300ns x 100kHz / 2.

Pfall = 119.1mW

where:

Pon = Rdson x Iout² x D (where D is the duty cycle

- at full charge, D can be approximated to 1)

Pon = 100m

Ω x 625mA². Pon = 39.1mW

c. The power lost in the fly back diode is Pdiode =

Vf x Iout(1-D) where D = Vout/Vcc = 6/12. D = 0.5

d. the power lost in the sense resistor (shunt resis-

tor) is Pshunt = Rsense x Iout²

Therefore,

Plosses = Pchip+Pswitch+Pdiode+Pshunt

= 72mW + 217.7mW + 219mW + 129mW

The yield (efficiency) is

η = 85.5%

The following table gives a tentative yield improve-

ment view following the choice of some external

components. Be aware that some of the following

choices have non negligeable cost effects on the

total application.

Improved yield - by changing the external components value one by one

Rsense

330m

Ω

220m

Ω

----

Iout

625mA

936mA

----

Vout (R1/R2)

6V

-

7.5V

-

-

-

Rdson

100m

Ω

-

-

140m

Ω

--

Ciss

0nF

-

-

0.85nF

-

-

PWM Freq

100kHz

-

-

-

50kHz

-

Free Wheel

0.7V

----

0.3V

Yield

85.5%

85.6%

88.9%

85.7%

87.3%

88.1%

Cost influence

-

=

=

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