CMOS logic gate is best because it can operate over a
wide supply voltage range (3V to 15V) and has enough
voltage swing to drive the internal Schmitt trigger shown
in Figure 5. For 5V applications, a TTL logic gate can be
used by simply adding an external pull-up resistor (see
Figure 6).
Capacitor Selection
While the exact values of CIN and COUT are noncritical,
good quality, low ESR capacitors such as solid tantalum
are necessary to minimize voltage losses at high cur-
rents. For CIN the effect of the ESR of the capacitor will
be multiplied by four, due to the fact that switch currents
are approximately two times higher than output current,
and losses will occur on both the charge and discharge
cycle. This means that using a capacitor with 1Ω of ESR
for CIN will have the same effect as increasing the output
impedance of the LTC1046 by 4Ω. This represents a sig-
nificant increase in the voltage losses. For COUT the effect
of ESR is less dramatic. COUT is alternately charged and
discharged at a current approximately equal to the output
current, and the ESR of the capacitor will cause a step
function to occur, in the output ripple, at the switch transi-
tions. This step function will degrade the output regula-
tion for changes in output load current, and should be
avoided. Realizing that large value tantalum capacitors
can be expensive, a technique that can be used is to par-
allel a smaller tantalum capacitor with a large aluminum
electrolytic capacitor to gain both low ESR and reasonable
cost. Where physical size is a concern some of the newer
chip type surface mount tantalum capacitors can be used.
These capacitors are normally rated at working voltages
in the 10V to 20V range and exhibit very low ESR (in the
range of 0.1Ω).
LV (Pin 6)
The internal logic of the LTC1046 runs between V+ and
LV (Pin 6). For V+ greater than or equal to 3V, an internal
switch shorts LV to GND (Pin 3). For V
+
less than 3V,
the LV pin should be tied to ground. For V+ greater than
or equal to 3V, the LV pin can be tied to ground or left
floating.
OSC (Pin 7) and BOOST (Pin 1)
The switching frequency can be raised, lowered or driven
from an external source. Figure 5 shows a functional dia-
gram of the oscillator circuit.
By connecting the BOOST (Pin 1) to V+, the charge and
discharge current is increased and, hence, the frequency
is increased by approximately three times. Increasing the
frequency will decrease output impedance and ripple for
higher load currents.
Loading Pin 7 with more capacitance will lower the fre-
quency. Using the BOOST pin in conjunction with exter-
nal capacitance on Pin 7 allows user selection of the fre-
quency over a wide range.
Driving the LTC1046 from an external frequency source
can be easily achieved by driving Pin 7 and leaving the
BOOST pin open, as shown in Figure 6. The output cur-
rent from Pin 7 is small, typically 15μA, so a logic gate
is capable of driving this current. The choice of using a
OSC
(7)
1046 F05
LV
(6)
BOOST
(1)
~14pF
I2I
I2I
V+
SCHMITT
TRIGGER
C2
V+
100k
OSC INPUT
REQUIRED FOR TTL LOGIC
–(V+)
1046 F06
1
2
3
4
8
7
6
5
V+
OSC
LV
VOUT
BOOST
CAP+
GND
CAP–
LTC1046
C1
NC
+
+
LTC1046
7
Rev. C
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APPLICATIONS INFORMATION
Figure 5. Oscillator Figure 6. External Clocking