LTC660 Linear Technology, LTC660 Datasheet - Page 7

• Manufacturer Part Number: LTC660
• Description: 100mA CMOS Voltage Converter
• Manufacturer: Linear Technology

APPLICATIONS
By connecting the BOOST pin (Pin 1) to V
discharge current is increased and, hence, the frequency
is increased by approximately four and a half times.
Increasing the frequency will decrease output impedance
and ripple for high load currents.
Loading Pin 7 with more capacitance will lower the fre-
quency. Using the BOOST (Pin 1) in conjunction with
external capacitance on Pin 7 allows user selection of the
frequency over a wide range.
Driving the LTC660 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 current from
Pin 7 is small, typically 1.1 A to 8 A, so a logic gate is
capable of driving this current. (A CMOS logic gate can be
used to drive the OSC pin.) For 5V applications, a TTL logic
gate can be used by simply adding an external pull-up
resistor (see Figure 6).
BOOST
(1)
(6)
LV
U
7.0I
7.0I
Figure 5. Oscillator
INFORMATION
V
U
+
I
I
W
18pF
OSC
(7)
+
, the charge and
SCHMITT
TRIGGER
LTC660 • F05
U
C1
Capacitor Selection
While the exact values of C1 and C2 are noncritical, good
quality, low ESR capacitors are necessary to minimize
voltage losses at high currents. For C1 the effect of the ESR
of the capacitor will be multiplied by four, due to the fact
the switch currents are approximately two times higher
than the output current and losses will occur on both the
charge and discharge cycle. This means using a capacitor
with 1
increasing the output impedance of the LTC660 by 4 .
This represents a significant increase in the voltage losses.
For C2 the effect of ESR is less dramatic. A C2 with 1 of
ESR will increase the output impedance by 1 . The size
of C2 and the load current will determine the output
voltage ripple. It is alternately charged and discharged at
a current approximately equal to the output current. This
will cause a step function to occur in the output voltage at
the switch transitions. For example, for a switching fre-
quency of 5kHz (one-half the nominal 10kHz oscillator
frequency) and C2 = 150 F with an ESR of 0.2 , ripple is
approximately 90mV with a 100mA load current.
+
NC
of ESR for C1 will have the same effect as
1
2
3
4
REQUIRED FOR TTL LOGIC
LTC660
Figure 6. External Clocking
8
7
6
5
+
100k
C2
–(V
+
)
V
LTC660
+
OSC INPUT
LTC660 • F06
7