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參數(shù)資料
| 型號(hào)： | LTC1429CS8-4 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類(lèi)： | 穩(wěn)壓器 |
| 英文描述： | Clock-Synchronized Switched Capacitor Regulated Voltage Inverter |
| 中文描述： | SWITCHED CAPACITOR REGULATOR, 700 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 0.150 INCH, PLASTIC, SO-8 |
| 文件頁(yè)數(shù)： | 10/12頁(yè) |
| 文件大小： | 275K |
| 代理商： | LTC1429CS8-4 |
10
LTC1429
going into very high frequency oscillation under such condi-
tions. 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 7). The resulting output voltage behaves
as if a resistor of value C
ESR
×
(I
PK
/I
AVE
)
was placed in series
with the output. To minimize this effect, output capacitor ESR
should be as low as possible or smaller value high frequency
bypass (typically a 0.1
μ
F ceramic) should be added in parallel
with the output capacitor.
APPLICATIO
S I
FOR
ATIO
U
W
U
U
Figure 7. Output Ripple with Low and High ESR Caps
LOW ESR
OUTPUT CAP
CLOCK
V
AVERAGE
V
SET
COMP1
OUTPUT
V
SET
V
OUT
HIGH ESR
OUTPUT CAP
V
AVERAGE
COMP1
OUTPUT
LTC1429 F07
V
OUT
Note that ESR in the flying caps will not cause the same
condition; in fact, it may actually improve the situation by
cutting the peak currents and lowering the amplitude of
the spike. More flying cap ESR is not necessarily better,
however; as soon as the RC time constant approaches half
of a clock period (the time the capacitors have to share
charge at full duty cycle) the output current capability of
the LTC1429 will begin to diminish. For 0.1
μ
F flying
capacitors and typical 700kHz external clock, this gives a
maximum total series resistance of:
)
700kHz
Most of this resistance is already provided by the internal
switches in the LTC1429 (especially in tripler mode). More
than 1
or 2
of ESR on the flying caps will start to affect
the regulation at maximum load.
=
/ 0.1
μ
F = 7.14
1
2
)
t
CLK
C
FLY
1
2
)
)
1
RESISTOR SELECTION
Resistor selection is easy with the fixed output versions of
the LTC1429; 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 1.24V at the ADJ pin with
respect to V
OUT
(Figure 8). The LTC1429 uses a positive
reference with respect to V
OUT
, not a negative reference
with respect to ground (Figure 4 shows reference connec-
tion). 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 14-pin adjustable parts include a built-in resistor
string which can provide an assortment of output voltages
by using different pin-strapping options at the RO, R1 and
R
ADJ
pins (Table 3). The internal resistors are roughly
124k, 226k, 100k and 50k (see Figure 4) giving output
options of –3.5V, –4V, –4.5V and –5V. The resistors are
carefully matched to provide accurate divider ratios, but
the absolute values can vary substantially from part to
part. It’s not a good idea to create a divider using an
external resistor and one of the internal resistors unless
the output voltage accuracy is not critical.
Table 3. Output Voltages Using the Internal Resistor Divider
PIN CONNECTIONS
ADJ - R
ADJ
ADJ - R
ADJ
, RO - GND
ADJ - R
ADJ
, R1 - RO
ADJ - R
ADJ
, R1-GND
ADJ - R1
ADJ - R0
ADJ - GND
OUTPUT VOLTAGE
–5.0V
–4.5V
–4.0V
–3.5V
–1.77V
–1.38V
–1.24V
There are some oddball output voltages available as well.
They are obtained by connecting ADJ to R0 or R1 and
shorting out some of the internal resistors. If one of them
gives you the output voltage you want, by all means use it!
LTC1429
GND
R1
6 (4)*
10 (5)*
11 (6)*
LTC1429CS
*(LTC1429CS8)
LTC1429 F07
V
OUT
= –1.24V
R2
(R1 + R2)
R2
ADJ
OUT
Figure 8. External Resistor Connections
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