DS1227 Datasheet, PDF (11/20 Pages) Dallas Semiconductor – KickStarter Chip

AUXILIARY BOOST SUPPLY CONFIGURATION Figure 8
100 µH (typ.) 1N5418
V_CCOAUX
270 µH (typ.)
+
+3V _–
200 µF (typ.)
DS1227
KICKSTARTER
OSCEXT
V_DCI
V_DCO
V_CC01
V_CC02
V_CC03
BOOST
GND
DS1227
The DC-DC converter’s output current is provided both
by the inductor and directly from the battery. If the load
draws less than the maximum current, the V_DCI n-chan-
nel MOSFET is turned on only often enough to keep the
output voltage at the desired level.
If the selected inductor has too high a value, the DS1227
will not be able to deliver the desired output power, even
with the MOSFET turned on for every oscillator cycle.
The available output power can be increased by either
raising the input voltage or lowering the inductance.
This causes the current to rise at a faster rate, and re-
sults in a higher peak current at the end of each cycle.
The available output power increases since it is propor-
tional to the square of the peak inductor current. The
maximum inductance therefore is:
L_MAX
+
V_IN ²
8 f_O P_L
since :
P_L
+
L
I_PK2
2
f_O
and :
I_PK
+
V_IN
2
f_O L
The required output power must include what is dissi-
pated in the forward drop of the catch diode and each of
the V_CCO1, V_CCO2, and V_CCO3 pass transistors. This
can be expressed as follows:
P_OUT = V_F I_OUT + (I_OUT12 R_ON1 + I_OUT22 R_ON2 +
I_OUT32 R_ON3 + I_OUT42 R_ON4) + V_OUT I_OUT
where:
I_OUT
= I_OUT1 + I_OUT2 + I_OUT3 + I_OUT4
If the inductance value is too low, the current at V_DCI
may rise above the maximum rating. The minimum al-
lowed inductor value is expressed by:
L_MIN
+
2
V_IN
f_O I_MAX
(I_MAX ) 450 mA)
TYPES OF INDUCTORS
The following is a brief discussion of various types of in-
ductors which may be typically used with the DS1227
Kickstarter to facilitate boost mode operation. Table 1
lists some typical manufacturers of these types of induc-
tors. Table 2 summarizes performance of the circuit for
various inductors.
Molded Inductors
These are cylindrically wound coils which look similar to
1-watt resistors. They have the advantages of low cost
and ease of handling, but have higher resistance, higher
losses, and lower power handling capability than other
types of inductors.
Potted Toroidal Inductors
A typical 1 mH, 0.82 ohm potted toroidal inductor (Dale
TE-3Q4TA) is 0.685 in diameter by 0.385 high and
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