AN45
2. Calculate the inductance, L, based on Equation 13
The voltage across the inductor during this time is equal
to V less the D1 diode voltage drop or approximately
assuming worst case 60% efficiency. Since inductors tend
to have tolerances in the range of ±5% to ±30%, the
minimum value of inductance must be equal to the
calculated value of the inductor. Vary Fs from 64 kHz to
128 kHz to obtain the desired inductance value.
BAT
V
. Since V
is greater than V , the rate of current
BAT
BAT DC
change is faster compared to the current slope during
on-time. The inductor current starts from I and
PK
descends to 0 A at t
.
OFFMAX
3. Calculate the period, T, for Fs and the corresponding value
for direct Register 92 in hexadecimal.
IPK
VBAT
-------------
L
----------------------
=
tOFFMAX
T
So the maximum off-time is as follows:
--------------
PWM Period Register 92 =
61 ns
Equation 14
IPK × L
-----------------
=
tOFFMAX
4. Calculate the maximum off-time and the corresponding
value for direct Register 93 in hexadecimal.
VBAT
Equation 10
t
(IPK × L) ⁄ VBAT
----------------------------------------
61 ns
--O----F----F---M----A----X-
Direct Register 93 =
=
The period of the switching frequency, Fs, is equal to
the reciprocal of the maximum on-time plus maximum
off-time:
61 ns
Equation 15
Selecting a DC-DC Converter Switching Transistor
1
1
-------------------------------------------------
---------------------------------------------------------------------------
Fs =
=
The switching transistor (Q7) on the typical application
circuit is shown in Figure 2 on page 3. This transistor is
turned on by the base drive current through Q8 while
R16 provides the discharge current path for Q7’s
base-emitter capacitor during turn-off. The capacitor,
C10, provides additional charge pump boost current
from the DCFF pin of the Si321x to turn Q7 off faster.
C10 with a value of 22 nF is sufficient for most
applications. R16 plays an important role in turning off
the Q7 transistor, but R16 also robs the Q7 base drive
current during the on-time. With a value of 200 Ω, R16
does an adequate job of turning Q7 off and only takes
3 mA from the base current during on-time.
t
ONMAX + tOFFMAX
IPK × L ⁄ VDC + IPK × L ⁄ VBAT
Substituting the above expression for the power
Equation 8:
L × IPK2 × Fs
---------------------------------
POUT = EFF × PIND = E
FF
2
Equation 11
And solving for I
:
PK
2POUT(VBAT + VDC
)
------------------------------------------------------
=
IPK
EFF × VBAT × VDC
Equation 12
0.6
IR16
----------
R16 =
Solving for L from Equation 11, the required inductance
is expressed as:
Equation 16
2POUT
EFF × IPK2 × Fs
Table 1 lists the requirements for the switching
transistor, Q7.
----------------------------------------
L =
Equation 13
Table 1. Switching Transistor Q7
The optimum switching frequency of the Si321x dc-dc
converter is between 64 kHz and 85 kHz. Faster
switching frequency is generally less efficient. This is a
common characteristic of the PNP switching element
and low-cost inductor magnetic material.
V
> |V
| + V
BAT DC
CEO
V
> V
CC
EBO
V
> |V
| + V + V
BAT CC DC
CBO
Power Inductor Selection
I
> I (maximum Inductor current)
PK
CMAX
Once output power (P
defined, the inductor can be selected as follows:
), V , and V
are clearly
BAT
OUT
DC
f > 100 MHz
T
1. Calculate IPK based on Equation 12 (assumed 60%
efficiency). This is the maximum current requirement for
the inductor.
Another critical specification is the transistor gain at
. The higher the transistor gain (h ), the less
base current is required to keep it in saturation during
I
CMAX
FE
Rev. 0.5
5
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