LTC1261LCMS8#TR Linear Technology, LTC1261LCMS8#TR Datasheet - Page 8

• Manufacturer Part Number: LTC1261LCMS8#TR
• Description: IC VOLT INVERT SW CAP ADJ 8MSOP
• Manufacturer: Linear Technology
• Type: Switched Capacitor (Charge Pump), Invertingr

Specifications of LTC1261LCMS8#TR

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: -1.23 ~ -5 V
• Current - Output: 20mA
• Frequency - Switching: 650kHz
• Voltage - Input: 2.7 ~ 5.25 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
LTC1261L
OUTPUT FILTERING
If extremely low output ripple (< 5mV) is required, addi-
tional output filtering is required. Because the LTC1261L
uses a high 650kHz switching frequency, fairly low value
RC or LC networks can be used at the output to effectively
filter the output ripple. A 10 series output resistor and a
3.3 F capacitor will cut output ripple to below 3mV (Figure
3). Further reductions can be obtained with larger filter
capacitors or by using an LC output filter.
CAPACITOR SELECTION
Capacitor Sizing
The performance of the LTC1261L is affected by the
capacitors to which it connects. The LTC1261L requires
bypass capacitors to ground for both the V
pins. The input capacitor provides most of LTC1261L’s
supply current while it is charging the flying capacitors.
This capacitor should be mounted as close to the package
as possible and its value should be at least ten times larger
than the flying capacitor. Ceramic capacitors generally
provide adequate performance. Avoid using a tantalum
capacitor as the input bypass unless there is at least a
0.1 F ceramic capacitor in parallel with it. The charge
pump capacitor is somewhat less critical since its peak
current is limited by the switches inside the LTC1261L.
Most applications should use a 0.1 F as the flying
capacitor value. Conveniently, ceramic capacitors are the
most common type of 0.1 F capacitor and they work well
here. Usually the easiest solution is to use the same
8
0.1 F
1 F
Figure 3. Output Filter Cuts Ripple Below 3mV
2
3
C1
C1
LTC1261L-4
+
–
GND
V
5V
CC
4
COMP
U
OUT
6
5
INFORMATION
U
100pF
+
W
3.3 F
10
1261L F03
+
3.3 F
CC
U
and OUT
V
OUT
= – 4V
capacitor type for both the input bypass capacitor and the
flying capacitor.
In applications where the maximum load current is well-
defined and output ripple is critical or input peak currents
need to be minimized, the flying capacitor value can be
tailored to the application. Reducing the value of the
flying capacitor reduces the amount of charge trans-
ferred with each clock cycle. This limits maximum output
current, but also cuts the size of the voltage step at the
output with each clock cycle. The smaller capacitor
draws smaller pulses of current out of V
limiting peak currents and reducing the demands on the
input supply. Table 1 shows recommended values of
flying capacitor vs maximum load capacity.
Table 1. Typical Max Load (mA) vs Flying Capacitor Value at
T
The output capacitor performs two functions: it provides
output current to the load during half of the charge pump
cycle and its value helps to set the output ripple voltage.
For applications that are insensitive to output ripple, the
output bypass capacitor can be as small as 1 F. Larger
output capacitors will reduce output ripple further at the
expense of turn-on time.
Capacitor ESR
Output capacitor Equivalent Series Resistance (ESR) is
another factor to consider. Excessive ESR in the output
capacitor can fool the regulation loop into keeping the
output artificially low by prematurely terminating the charg-
ing cycle. As the charge pump switches to recharge the
output a brief surge of current flows from the flying
capacitors to the output capacitor. This current surge can
be as high as 100mA under full load conditions. A typical
A
= 25 C, V
CAPACITOR
VALUE ( F)
OUT
FLYING
0.047
0.033
0.022
0.01
0.1
= – 4V
MAX LOAD (mA)
V
CC
20
15
10
5
1
= 5V
CC
as well,