LTC1261LCMS8#TR Linear Technology, LTC1261LCMS8#TR Datasheet - Page 9

• 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
3.3 F tantalum capacitor has 1 or 2 of ESR; 100mA
2 = 200mV. If the output is within 200mV of the set point
this additional 200mV surge will trip the feedback com-
parator and terminate the charging cycle. The pulse dissi-
pates quickly and the comparator returns to the correct
state, but the RS latch will not allow the charge pump to
respond until the next clock edge. This prevents the charge
pump from going into very high frequency oscillation
under such conditions but it also creates an output error
as the feedback loop regulates based on the top of the
spike, not the average value of the output (Figure 4). The
resulting output voltage behaves as if a resistor of value
C
avoid this nasty sequence of events, connect a 0.1 F
ceramic capacitor in parallel with the larger output capaci-
tor. The ceramic capacitor will “eat” the high frequency
spike, preventing it from fooling the feedback loop, while
the larger but slower tantalum or aluminum output capaci-
tor supplies output current to the load between charge
cycles.
Note that ESR in the flying capacitor will not cause the
same condition; in fact, it may actually improve the situa-
tion by cutting the peak current and lowering the ampli-
tude of the spike. However, more flying capacitor ESR is
not necessarily better. As soon as the RC time constant
ESR
OUTPUT CAP
OUTPUT CAP
Figure 4. Output Ripple with Low and High ESR Capacitors
HIGH ESR
LOW ESR
CLOCK
(I
PK
/I
AVE
) was placed in series with the output. To
U
V
V
OUT
OUT
INFORMATION
U
W
U
V
V
AVERAGE
COMP1
OUTPUT
V
V
AVERAGE
COMP1
OUTPUT
SET
OUT
SET
OUT
1261L F04
approaches half of a clock period (the time the capacitors
have to share charge at full duty cycle) the output current
capability of the LTC1261L starts to diminish. For a 0.1 F
flying capacitor, this gives a maximum total series resis-
tance of:
Most of this resistance is already provided by the internal
switches in the LTC1261L. More than 1 or 2 of ESR on
the flying capacitors will start to affect the regulation at
maximum load.
RESISTOR SELECTION
Resistor selection is easy with the fixed output voltage
versions of the LTC1261L— no resistors are needed!
Selecting the right resistors for the adjustable parts is only
a little more difficult. A resistor divider should be used to
divide the signal at the output to give 1.23V at the ADJ pin
with respect to V
positive reference with respect to V
reference with respect to ground (Figure 1 shows the
reference connection). Be sure to keep this in mind when
connecting the resistors! If the initial output is not what
you expected, try swapping the two resistors.
The LTC1261L can be internally configured for other
fixed output voltages. Contact the Linear Technology
Marketing department for details.
1
2
C
t
CLK
FLY
Figure 5. External Resistor Connections
LTC1261L
=
1
2
GND
OUT
OUT
ADJ
650kHz
1
(Figure 5). The LTC1261L uses a
4
5
6
R1
R2
/ 0.1 F = 7.7
V
OUT
= –1.23V
OUT
LTC1261L
, not a negative
R1 + R2
R2
1261L F05
9