AN45

Rev. 0.5

9

Step 6: Select the Base Drive Circuit

Q8 transistor requirement:

The general purpose 2222 transistor meets all of the

above requirements.

Calculate the value for R17:

Base current drive requirement for Q7

From Equation 17:

From Equation 18:

Step 7: Under Voltage Circuit Design

Set under voltage lock-out to be 20% lower than the

From Equation 20:

Step 8: Overcurrent Protection

From Equation 21:

Step 9: Output Overvoltage Protection

VBAT max = 78 V (during ringing). See Table 2 for

voltage ratings.

If VCC = 5 V, then

MOSFET/Transformer Design

DC-DC Converter

The MOSFET/transformer dc-dc converter solution

offers higher power efficiency than the BJT/inductor

solutions and is the preferred solution for applications

The transformer dc-dc converter circuit is shown in

Figure 6. R16, R17, Q7, and Q8 are eliminated. The M1

MOSFET is the main power-switching component in this

design. The Si321xM version of the ProSLIC is used to

directly drive the M1 MOSFET using the DCFF pinout.

V

CEO

V

CC

V

DC

+

510

+

15 V

==

=

V

EBO

V

CC

5V

==

V

CBO

V

CC

V

DC

+

510

+

15 V

==

=

Speed: f

T

200 MHz

>

I

BQ7

1.3

I

MAX

×

H

FE

---------------------------

1.3

0.98

×

100

--------------------------

12.74 mA

==

=

R17

V

CC

0.6

–

I

BQ7

0.6 R16

⁄

+

------------------------------------------

275

Ω

==

V

UNDER

1–20%

()VDC

8 V

==

R19

V

UNDER

0.8

–

120

µA

------------------------------------





4500

–

56K

==

R19

R

=

20

56K

=

I

OVERLOAD

1.2

I

MAX

×

1.176 A

==

R18

10.5

µA

4500

R19

+

()

×

I

OVERLOAD

---------------------------------------------------------------

0.5

Ω

==

Table 2. Component Voltage Rating

Components

Voltage

Parameter

Voltage

Rating

C9, C26, C3, C4, C5, C6

98.5

Q1–Q4 (2N5401)

150

Q5, Q6

150

Q7

R28

5.55

+

() V

148

µA

----------------------------

37.4k

==

R29

85 V

148

µA

--------------------

574k

==