參數(shù)資料
型號(hào)： LT1939IDD#TRPBF
廠商： Linear Technology
文件頁(yè)數(shù)： 14/24頁(yè)
文件大?。?/td> 349K
描述： IC REG DL BUCK/LINEAR 12DFN
標(biāo)準(zhǔn)包裝： 2,500
拓?fù)洌?/td> 降壓（降壓）（1），線性（LDO）（1）
功能： 任何功能
輸出數(shù)： 2
頻率 - 開關(guān)： 500kHz ~ 2.4MHz
電壓/電流 - 輸出 1： 0.8 V ~ 24.25 V，2A
電壓/電流 - 輸出 2： 0.8 V ~ 24.25 V，1A
帶 LED 驅(qū)動(dòng)器： 無
帶監(jiān)控器： 無
帶序列發(fā)生器： 無
電源電壓： 3 V ~ 25 V
工作溫度： -40°C ~ 125°C
安裝類型： 表面貼裝
封裝/外殼： 12-WFDFN 裸露焊盤
供應(yīng)商設(shè)備封裝： 12-DFN
包裝： 帶卷 (TR)
LT1939
14
1939f
surge currents when the input source is applied, tantalum
capacitors should be surge rated. The manufacturer may
also recommend operation below the rated voltage of the
capacitor. Be sure to place the 1糉 ceramic as close as
possible to the V
IN
 and GND pins on the IC for optimal
noise immunity.
A  nal caution regarding the use of ceramic capacitors for
input bypassing. A ceramic input capacitor can combine
with stray inductance to form a resonant tank circuit. If
power is applied quickly (for example, by plugging the
circuit into a live power source) this tank can ring, doubling
the input voltage and damaging the LT1939. The solution is
to either clamp the input voltage or dampen the tank circuit
by adding a lossy capacitor in parallel with the ceramic
capacitor. For details see Application Note 88.
Output Capacitor Selection
Typically step-down regulators are easily compensated with
an output crossover frequency that is 1/10 of the switch-
ing frequency. This means that the time that the output
capacitor must supply the output load during a transient
step is ~2 or 3 switching periods. With an allowable 5%
drop in output voltage during the step, a good starting
value for the output capacitor can be expressed by:
 
C
VOUT1
=
MaxLoadStep
Frequency"0.05"V
OUT
1
Example:
  V
OUT1
 = 3.3V, Frequency = 1MHz, Max Load Step = 2A
 
C
VOUT1
=
2
1MHz"0.05"
3.3
=12糉
The calculated value is only a suggested starting value.
Increase the value if transient response needs improvement
or reduce the capacitance if size is a priority. The output
capacitor  lters the inductor current to generate an output
with low voltage ripple. It also stores energy in order to
satisfy transient loads and to stabilize the LT1939s control
loop. The switching frequency of the LT1939 determines
the value of output capacitance required. Also, the current
mode control loop doesnt require the presence of output
capacitor series resistance (ESR). For these reasons, you
are free to use ceramic capacitors to achieve very low
output ripple and small circuit size. Estimate output ripple
with the following equations:
 
V
RIPPLE
=
I
L
8"Frequency"C
OUT
1
  For ceramic capacitors and,
  V
RIPPLE
 = 擨
L
 " ESR
  For electrolytic (tantalum and aluminum)
where 擨
L
 is the peak-to-peak ripple current in the
inductor.
The RMS content of this ripple is very low, and the RMS
current rating of the output capacitor is usually not of
concern.
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
to the regulation voltage. For a 5% overshoot, this require-
ment becomes:
 
C
OUT1
>10"L
I
LIM
V
OUT
1
2
Finally, there must be enough capacitance for good transient
performance. The last equation gives a good starting point.
Alternatively, you can start with one of the designs in this
data sheet and experiment to get the desired performance.
This topic is covered more thoroughly in the section on
loop compensation.
The high performance (low ESR), small size and robust-
ness of ceramic capacitors make them the preferred type
for LT1939 applications. However, all ceramic capacitors
are not the same. As mentioned above, many of the high
value capacitors use poor dielectrics with high tempera-
ture and voltage coef cients. In particular, Y5V and Z5U
types lose a large fraction of their capacitance with ap-
plied voltage and temperature extremes. Because the loop
stability and transient response depend on the value of
C
OUT
, you may not be able to tolerate this loss. Use X7R
and X5R types. You can also use electrolytic capacitors.
APPLICATIONS INFORMATION
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