DS1227 Datasheet, PDF (13/20 Pages) Dallas Semiconductor – KickStarter Chip

DS1227
INDUCTOR SELECTION FOR COMMON DESIGNS Table 2
V_IN
V_DCO
I_OUT
EFF.
(V)
(V)
(mA)
(%)
INDUCTOR
PART #
uH
2
5
5
78
CB 6860-21
470
2
5
10
74
G 1B253
250
2
5
15
61
G 1B103
100
3
5
25
82
CB 6860-21
470
3
5
40
75
CB 7070-29
220
Note:
CB = Cadell-Burns, NY (516) - 746 -2310
G = Gowanda Electronics Corp., NY (716) - 532-2234
Other manufacturers listed in Table 1.
Ohms
0.4
0.44
0.25
0.4
0.55
OUTPUT FILTER CAPACITOR
In boost regulation mode, the DS1227’s output voltage
ripple on V_DCO has two components, with approximate-
ly 90^o phase difference between them. One component
is created by the change in the capacitor’s stored
charge with each output pulse. The other ripple compo-
nent is the product of the capacitor’s charge/discharge
current and its ESR (Effective Series Resistance). With
low cost aluminum electrolytic capacitors, the ESR pro-
duced ripple is generally larger than that caused by the
change in charge.
V_ESR
+ I_PK x ESR
+
V_IN
2Lf_O
x ESR (Volts p * p)
Where V_IN is the coil input voltage, L is its inductance, f
is the oscillator frequency, and ESR is the equivalent se-
ries resistance of the filter capacitor.
The output ripple resulting from the change in charge on
the filter capacitor is:
V_dQ
+
Q
C
where,
Q
+
t_DIS
x I_peak
2
and,
I_peak
+
t_CHG x
L
V_IN
V_dQ
+
V_IN
x
t_CHG
2LC
x
t_DIS
Where t_CHG and t_DIS are the charge and discharge
times for the inductor 1/2 f_O can be used for nominal cal-
culations).
High quality aluminum or tantalum filter capacitors will
minimize output ripple, even if smaller capacitance val-
ues are used. Best results at reasonable cost are typi-
cally achieved in the 100 to 500 µF range, in parallel with
a 0.1 µF ceramic capacitor.
OSCEXT FUNCTIONS
The OSCEXT pin is connected to the internal 40 KHz
oscillator (nominal frequency). When Boost mode is en-
abled (BOOST = 0) and the DC-to-DC converter is run-
ning, OSCEXT is active at the same time whenever the
error comparator is switched high, i.e., whenever the in-
ternal oscillator is enabled to the gate of the V_DCI driver.
In this configuration it may be used to drive an auxiliary
switch mode boost regulator as shown in Figure 8. In
this circuit, OSCEXT drives an external NMOS switch
with its drain pin connected to an additional inductor and
filter capacitor as well as an external catch diode. The
amount of supply current which can be realized at the
+5V output is determined by the power ratings of the ex-
ternal components. Through proper selection of the
these components, increased supply current can be
realized than is possible using the Kickstarter’s internal
V_DCI driver and catch diode.
When the Pass-Through mode is enabled (BOOST = 1)
and at least one of the V_CCO outputs is switched on, the
OSCEXT pin will be continuously driven with the 40 KHz
frequency. In this configuration this pin could potentially
be used to generated negative or doubled voltages as
shown in Figure 9.
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