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参数资料
| 型号： | LT1767EMS8#TR-3.3 |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 1500 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, MSOP-8 |
| 文件页数： | 14/16页 |
| 文件大小： | 232K |
| 代理商： | LT1767EMS8#TR-3.3 |
7
LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
APPLICATIONS INFORMATION
WU
U
FB RESISTOR NETWORK
If an output voltage of 1.8V, 2.5V, 3.3V or 5V is required,
the respective fixed option part, -1.8, -2.5, -3.3 or -5,
should be used. The FB pin is tied directly to the output; the
necessary resistive divider is already included on the part.
For other voltage outputs, the adjustable part should be
used and an external resistor divider added. The sug-
gested resistor (R2) from FB to ground is 10k. This
reduces the contribution of FB input bias current to output
voltage to less than 0.25%. The formula for the resistor
(R1) from VOUT to FB is:
R
RV
RA
OUT
1
21 2
12
2 0 25
=
()
.
.( .
)
applications. Y5V or similar type ceramics can be used
since the absolute value of capacitance is less important
and has no significant effect on loop stability. If operation
is required close to the minimum input required by the
output of the LT1767, a larger value may be required. This
is to prevent excessive ripple causing dips below the
minimum operating voltage resulting in erratic operation.
If tantalum capacitors are used, values in the 22
F to
470
F range are generally needed to minimize ESR and
meet ripple current and surge ratings. Care should be
taken to ensure the ripple and surge ratings are not
exceeded. The AVX TPS and Kemet T495 series are surge
rated. AVX recommends derating capacitor operating
voltage by 2:1 for high surge applications.
OUTPUT CAPACITOR
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
triangular, with an RMS value given by:
I
VV
V
Lf V
RIPPLE RMS
OUT
IN
OUT
IN
( ) =
()
()
()( )( )
029
.
The LT1767 will operate with both ceramic and tantalum
output capacitors. Ceramic capacitors are generally cho-
sen for their small size, very low ESR (effective series
resistance), and good high frequency operation, reducing
output ripple voltage. Their low ESR removes a useful zero
in the loop frequency response, common to tantalum
capacitors. To compensate for this, the VC loop compen-
sation pole frequency must typically be reduced by a factor
of 10. Typical ceramic output capacitors are in the 1
F to
10
F range. Since the absolute value of capacitance de-
fines the pole frequency of the output stage, an X7R or X5R
type ceramic, which have good temperature stability, is
recommended.
Tantalum capacitors are usually chosen for their bulk
capacitance properties, useful in high transient load appli-
cations. ESR rather than absolute value defines output
ripple at 1.25MHz. Typical LT1767 applications require a
tantalum capacitor with less than 0.3
ESR at 22F to
500
F, see Table 2.
Figure 2. Feedback Network
–
+
1.2V
VSW
VC
GND
1767 F02
R1
R2
10k
OUTPUT
ERROR
AMPLIFIER
FB
LT1767
+
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple this causes at the input of LT1767 and force the
switching current into a tight local loop, thereby minimiz-
ing EMI. The RMS ripple current can be calculated from:
II
V
RIPPLE RMS
OUT
IN
OUT
IN
( ) =
()/ 2
Higher value, lower cost ceramic capacitors are now
available in smaller case sizes. These are ideal for input
bypassing since their high frequency capacitive nature
removes most ripple current rating and turn-on surge
problems. At higher switching frequency, the energy stor-
age requirement of the input capacitor is reduced so
values in the range of 1
F to 4.7F are suitable for most
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