AN45 SILABS [Silicon Laboratories], AN45 Datasheet - Page 8

• Manufacturer Part Number: AN45
• Description: DESIGN GUIDE FOR THE Si3210/15/16 DC-DC CONVERTER
• Manufacturer: SILABS [Silicon Laboratories]

=
AN45
Step 2: Selecting Output Power Requirement
Set up for power optimization in the active off-hook
mode:
From Equation 5:
From track 1 Equation 6:
Conclusion:
The 2.9 W ringing power is the worst-case power
requirement
requirement is much lower (.46 W). The ringing power is
used for the design of the dc-dc converter.
Step 3: Define Input Requirement for the 12 V DC
From Equation 8:
Experiments with the unregulated 12 V source showed
that the actual V
current equal to .36 A. The adjusted V
follows:
The total current drew on the 12 V unregulated supply is
.48 A, which is well within the 12 V unregulated
maximum specification of .75 A.
8
24.5 mA 3
I
BAT
P
=
OFFHOOK
I
LIM
(
I
I
IN
IN
+ +
+
9
=
I
I
I
Track
V
V
because
=
BJTBIAS
LIM
BJTBIAS
CM
OV
-------------------------- -
V
=
-------------------------- -
V
DC
20 mA 2 2000
IN
IN
P
I
P
=
BAT
=
=
OUT
×
voltage drops down to 10 V at input
OUT
×
=
20 mA Register 65
60%
9 V Register 66
3 V Register 73
60%
V
+
(
=
1 Register 71
(
=
V
DC
(
0.6
24.5 mA
CM
(
4 mA Register 66
the
(
×
(
=
=
=
(
+
+
-------------------- -
10
-------------------- -
12 0.6
10 V
V
80 I
active
2.9
OV
(
2.9
×
×
×
(
0.6
0.045
LIM
+
I
LIM
=
+
)
=
)
)
I
0.48 A
off-hook
+
BJTBIAS
DC
0.4 A
×
)
160
R
LINE
and I
)
)
)
)
=
)
) 5100
IN
⁄
.46 W
power
is as
Rev. 0.5
Step 4: Selecting the Power Inductor
Calculate the I
Calculate the L1 Inductance from Equation 13:
(This frequency was selected to round up the inductor
value to 100 µH.)
From Equation 14:
From Equation 15:
Step 5: Selecting the Q7 Switching Transistor
Transistor requirement:
The Zetex FZT955 bipolar transistor meets all of the
above requirements and its HFE gain at I
100.
Let I
discharge.
From Equation 16:
Delay Register 93
R16
Period Register 92
V
CBO
= 3 mA for adequate Q7 base capacitor
I
PK
>
V
V
L
CEO
PK
=
BAT
=
2P
------------------------------------------------------ -
R16
using Equation 12:
T
---------------------------------------- -
E
>
E
+
FF
Speed: f
OUT
FF
V
V
I
=
CMAX
V
Fs
EBO
BAT
=
=
CC
2P
×
×
--------------- -
89500
=
(
I
21
V
t
---------------------- -
V
PK
=
OFFMAX
1
-------------- -
0.003
OUT
+
61 nS
+
>
BAT
BAT
0.6
=
>
89.5 kHz
V
2
=
V
V
I
DC
×
11.2 µS
-------------------- -
T
DC
CC
PK
15 H
=
61 nS
×
+
>
Fs
V
=
11.2 µS
=
=
V
100 MHz
=
=
DC
DC
=
78
1.14 A
200 Ω
78
8 V
=
0.98
-----------------------------------------------------
)
100 µH
+
+
=
=
5
10
183
+
1.14 A
×
=
10
(
61 nS
100 µH
PK
88 V
=
=
= 1.14 A is
b7 H
93 V
) 75
⁄