STEVAL-ISA059V1 STMicroelectronics, STEVAL-ISA059V1 Datasheet - Page 10

• Manufacturer Part Number: STEVAL-ISA059V1
• Description: DEMO BOARD BASED ON TSM108
• Manufacturer: STMicroelectronics
• Type: DC/DC Switching Converters, Regulators & Controllersr

Specifications of STEVAL-ISA059V1

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 6V
• Current - Output: 800mA
• Voltage - Input: 12V
• Regulator Topology: Buck
• Board Type: Fully Populated
• Utilized Ic / Part: TSM108
• Input Voltage: 12 V
• Output Voltage: 6 V
• Product: Power Management Modules
• Silicon Manufacturer: ST Micro
• Silicon Core Number: TSM108
• Kit Application Type: Power Management - Voltage Regulator
• Application Sub Type: DC/DC Converter
• Kit Contents: Board
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Frequency - Switching: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• For Use With/related Products: TSM108
• Other names: 497-8716

TSM108
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).
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 = 72mΩ
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.
Improved yield - by changing the external components value one by one
10/13
Rsense
Iout
Vout (R1/R2)
Rdson
Ciss
PWM Freq
Free Wheel
Yield
Cost influence
Mosfet: Pchannel Mosfet: Rdson = 100mΩ,
Ciss = 1nF.
Driver: TSM108
PWM frequency: 100kHz
Free wheel diode: Vf = 0.7V
Shunt: Rsense = 330mΩ
Eq3: η = Pout/Pin
Pin = Pout + Plosses
100kHz
330mΩ
100mΩ
625mA
85.5%
0.7V
0nF
6V
-
220mΩ
936mA
85.6%
=
-
-
-
-
-
88.9%
7.5V
=
-
-
-
-
-
-
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 = 100mΩ 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
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.
Pswitch = Prise + Pfall + Pon
Pswitch = 217.7mW
Pdiode=219mW
Pshunt = 129mW
Plosses = 638mW
Pout / Pin = 3.75 / (3.75 + 0.638) = 85.5%
140mΩ
0.85nF
85.7%
<
-
-
-
-
-
η = 85.5%
50kHz
87.3%
>
-
-
-
-
-
-
88.1%
0.3V
>>
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-
-
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