DS1227 Datasheet, PDF (9/20 Pages) Dallas Semiconductor – KickStarter Chip

DS1227
BOOST MODE OPERATION
The DS1227 Kickstarter incorporates all of the neces-
sary control and power switching functions required for
its +3V to +5V step-up DC-to-DC converter. These func-
tions include a bandgap reference, oscillator, voltage
comparator, catch diode and an N-channel MOSFET.
The only external components required are an output fil-
ter capacitor and a low cost inductor. The block diagram
shown in Figure 7 illustrates the DC-to-DC converter.
When kickstarting occurs from an initial powered down
state (i.e., V_CCO1, V_CCO2, and V_CCO3 turned off), an in-
ternal start sequence is initiated within the DS1227.
During this sequence, the V_CCO1 output remains shut
off and the BOOST pin is sampled in order to determine
if the DS1227 is configured for boost mode operation. If
BOOST is low, then boost mode operation is enabled
and the DC-to-DC converter is started.
The internal DC-to-DC converter is started by enabling
the on-chip 40 KHz oscillator. It then begins to build up
the voltage on the V_DCO filter capacitor. Internal counter
logic insures that the DC-to-DC converter stays in start
mode for a minimum of six clock periods (nominally
150 µs @ 40 KHz). After this initial delay time, the V_DCO
output is monitored by the internal Error Comparator as
it slews up to V_DCON. As long as the V_DCO voltage re-
mains below the preset value, the Error Comparator will
be switched high and the internal 40 KHz oscillator will
be connected to the gate of the V_DCI driver.
The V_DCI driver is a large N-channel MOSFET with a
typical ON resistance of less than 4 Ohms and is capa-
ble of supplying a peak current of 450 mA. The output
device is turned on during each ON half-cycle gener-
ated by the internal square-wave oscillator, and is
turned off during each OFF half-cycle. During each ON
half-cycle, the current through the inductor rises linearly,
storing energy in the coil. When the output device is
turned off, the external inductor’s magnetic field col-
lapses, and the voltage across the inductor reverses
sign. The voltage at V_DCI then rises until the internal
diode is forward biased, delivering power to the V_DCO
output. The converter is thereby powered from its own
V_DCO output. This is often referred to as “bootstrapped”
operation, since the circuit figuratively “lifts” itself up. In
order to guarantee that the Kickstarter can bootstrap it-
self up to operating voltage, the V_DCI voltage must be at
the minimum level of V_DCISU as listed in the DC charac-
teristics section of this data sheet.
When the voltage on V_DCO rises to the V_DCON thresh-
old, the internal signal called “HIENUF” will be active
and the V_CCO1 PMOS device is switched on. As noted
above, internal circuitry insures that this device will not
be switched on for a minimum of 6 clock cycles from the
time that the DC-to-DC converter is started. However,
since the recommended values for the external LC com-
ponents result in a time constant which is much longer
than six cycles, the actual slew rate will in practice be
much longer than this delay time.
If loading of the V_CCO outputs causes V_DCO to drop be-
low V_DCOFF the DS1227 will deactivate HIENUF and
the V_CCO1 PMOS device as well as the other V_CCO
PMOS devices will be switched off. The V_DCO voltage
will then be monitored for the V_DCON trip point before re-
connecting the load. As a result, the power control regu-
lation loop could oscillate between these two states until
the V_CCO1 node had sufficient charge to remain above
the V_DCOFF threshold. To prevent this from occurring,
the value of the filter capacitor must be sufficiently large.
For large capacitive loads on V_CCO1 the output may dip
below V_DCOFF as a result of charge sharing and a larger
regulation capacitor at V_DCO may be required. For large
resistive loads the inductance and capacitance values
may need to be adjusted using a smaller inductor value
and large capacitance. In order not to violate the peak
V_DCI current it may be necessary to use the external os-
cillator OSCEXT to drive an additional switchmode
boost regulator, as shown in Figure 8.
Following the above described start sequence, normal
boost operation is performed by the converter. V_DCO
output voltage is constantly monitored by the error com-
parator. When V_DCO voltage drops below the preset val-
ue, the error comparator switches high and connects
the internal 40 KHz oscillator to the gate of the V_DCI out-
put driver. When the output voltage reaches the desired
level, the error comparator inhibits the V_DCI output driv-
er until the load on V_CCO1 discharges the output filter ca-
pacitor to less than the desired output level.
INDUCTOR SELECTION
The available output current from the Kickstarter’s
on-chip DC-DC boost converter is a function of the input
voltage, external inductor value, output voltage and the
operating frequency. For most applications, the inductor
is the only design variable since the internal oscillator is
preset to a fixed value of 40 KHz. The proper inductor
must have the following characteristics:
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