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参数资料
| 型号： | LT1680IN |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | High Power DC/DC Step-Up Controller |
| 中文描述： | SWITCHING CONTROLLER, 200 kHz SWITCHING FREQ-MAX, PDIP16 |
| 封装： | 0.300 INCH, PLASTIC, DIP-16 |
| 文件页数： | 9/16页 |
| 文件大小： | 322K |
| 代理商： | LT1680IN |
9
LT1680
OPERATIO
Slope Compensation
For duty cycles greater than 50%, slope compensation is
required to prevent current mode duty cycle instability in
the regulator control loop. The LT1680 employs internal
slope compensation that is adequate for most applica-
U
tions. However, if additional slope compensation is de-
sired, it is available through the SL/ADJ pin. Excessive
slope compensation will cause reduction in maximum
load current capability and is generally not desirable.
APPLICATIO
S I
FOR
ATIO
U
R
SENSE
Selection for Input Current Limit
R
SENSE
generates a voltage that is proportional to the
inductor current for use by the LT1680 current sense
amplifier. The value of R
SENSE
is based on the required
input current. The average current limit function has a
typical threshold of 120mV/R
SENSE
, or:
R
SENSE
= 120mV/I
LIMIT
Operation with V
SENSE
common mode voltage below 4.5V
may slightly degrade current limit accuracy. See Average
Current Limit Threshold Tolerance vs Common Mode
Voltage in the Typical Performance Characteristics sec-
tion for more information.
W
U
U
Output Voltage Programming
Output voltage is programmed through a resistor feed-
back network to the V
FB
pin (Pin 7) on the LT1680. This pin
is the inverting input of the error amplifier, which is
internally referenced to 1.25V. The divider is ratioed to
provide 1.25V at the V
FB
pin when the output is at its
desired value. Output voltage is thus set following the
relation:
V
OUT
= 1.25V(1 + R2/R1)
when an external resistor divider is connected to the
output as shown in Figure 1.
If high value feedback resistors are used, the input bias
current of the V
FB
pin (1
μ
A maximum) could cause a slight
increase in output voltage. A Thevenin resistance at the
V
FB
pin of <5k is recommended.
Oscillator Components R
CT
and C
CT
The LT1680 oscillator creates a modified sawtooth at its
timing node (C
T
) with a slow charge, rapid discharge
characteristic. The discharge time (t
DISCH
) corresponds to
the minimum off time of the PWM controller. This limits
maximum duty cycle (DC
MAX
) to:
DC
MAX
= 1 – (t
DISCH
)(f
O
)
This relation corresponds to the minimum value of the
timing resistor (R
CT
), which can be determined according
to the following relation (R
CT
vs DC
MAX
graph appears in
the Typical Performance Characteristics section):
R
CT(MIN)
≈
[(0.8)(10
–3
)(1 – DC
MAX
)]
–1
Values for R
CT
> 15k yield maximum duty cycles above
90%. Given a timing resistor value, the value of the timing
capacitor (C
CT
) can then be determined for desired oper-
ating frequency (f
O
) using the relation:
C
f
R
(
R
CT
O
CT
CT
≈
(
) (
)
)
+
( )
(
)
1
100 10
/1 85
1.
2 5 10
.
3 375
.
9
3
/
–
–
/
–
–
A plot of Operating Frequency vs R
CT
and C
CT
is shown in
Figure 2. Typical 100kHz operational values are C
CT
=
1000pF and R
CT
= 16.9k.
LT1680
V
OUT
V
FB
6
7
R1
R2
1680 F01
SGND
Figure 1. Programming LT1680 Output Voltage
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