ADVANCED INFORMATION
DESIGN CONSIDERATIONS
TK654xx
INDUCTOR SELECTION
shutdown threshold which occurs at a voltage lower than
theLBIthreshold.BoththeLBIthresholdandtheshutdown
threshold can be set utilizing a single resistive divider.
One of the positive features of the TK654xx is the ability to
limit the magnitude of the short circuit current without the
need for external current sensing resistors. Since the
maximum on-time duration is limited, and the converter
alwaysoperatesindiscontinuousconductionmode(DCM),
the maximum load current is essentially dependent upon
the selection of the inductor value. The following table
defines the maximum load current as a function of the
inductor value.
For the following explanation, please refer to Figure 3.
When the battery voltage (V ) is very high, the current
IN
from the LBI input pin is essentially zero. The voltage on
the LBI input pin is simply a resistively-divided version of
the V pin.
IN
V
IN
300 k
R
R
1
2
L
MAX LOAD
305 mA
140 mA
65 mA
LBO
LBI/SB
GND
LBO
OUTPUT
47 µH
100 µH
220 µH
470 µH
V
OUT
SW
V
IN
20 mA
FIGURE 3: LOW BATTERY MONITOR
OUTPUT FILTER CAPACITOR SELECTION
As the voltage on the LBI input pin drops and passes
through the bandgap voltage (1.22 V), the LBI input pin will
begin to source current. At this same point, the LBO output
will be asserted. The assertion of the LBO output is
typically used as an early warning, indicating that battery
failure is approaching.
The purpose of the filter capacitor on the output is to
temporarily provide current to the load as inductor current
is ramping upward or downward. In general, larger
capacitors will reduce the magnitude of the ripple.
INPUT FILTER CAPACITOR SELECTION
As the voltage on the V pin continues to drop, the current
IN
sourced by the LBI input pin will continue to linearly
increase with the drop-in supply voltage. The current from
the LBI input pin will attempt to keep the voltage pin at the
same potential as the bandgap reference (1.22 V).
Throughout this region, the LBO output is continuously
asserted.
Typically, the power supply or battery source connected to
the V pin contains an inductive and resistive component.
IN
Since the current draw from the input supply varies with
time, such a scenario would exhibit ripple on the input
supply. Adding a bypass capacitor between the V pin
IN
and ground, in some cases, will dramatically decrease
rippleontheinputsupply.Aninputsupplybypasscapacitor
in the range of 1 to 10 µF will work well for most converter
applications up to 100 mA. Bypass capacitors on the input
pin should exhibit relatively good ESR characteristics.
The current sourced from the LBI input pin is internally
limited to a maximum of approximately 720 nA. As
mentioned above, the current from the LBI input pin
continues to increase as the supply voltage drops. When
the current from the LBI input pin reaches the 720 nA
threshold,theTK654xxwillturnoff.Thisessentiallyprovides
a second threshold, less than the LBI/SB threshold, where
the system will turn off.
LOW BATTERY MONITOR
TheTK654xxutilizesaproprietarymethodforsensingand
handling low battery voltages. Using a single input pin and
resistive divider, the TK654xx not only provides an early
warning system of a low battery, it also provides a later
September 1999 TOKO, Inc.
Page 13
TOKO [ TOKO, INC ]
