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參數(shù)資料
| 型號(hào)： | LTC1779 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 250mA Low Dropout Current Mode Step-Down DC/DC Converter in SOT-23(2.5V輸出,250mA電流模式，低壓差,步降DC/DC變換器（SOT-23封裝）) |
| 中文描述： | 250mA的低壓差電流模式降壓DC / DC轉(zhuǎn)換器采用SOT - 23（2.5V的輸出，250mA的電流模式，低壓差，步降直流/直流變換器（采用SOT - 23封裝）） |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 174K |
| 代理商： | LTC1779 |
9
LTC1779
The MOSFET R
DS(ON)
plus R
SENSE
multiplied by duty
cycle can be summed with the resistances of L and
R
SENSE
to obtain I
2
R losses.
4. The output diode is a major source of power loss at
high currents and gets worse at high input voltages.
The diode loss is calculated by multiplying the forward
voltage times the diode duty cycle multiplied by the
load current. For example, assuming a duty cycle of
50% with a Schottky diode forward voltage drop of
0.4V, the loss increases from 0.5% to 8% as the load
current increases from 0.5A to 2A.
5. Transition losses apply to the internal MOSFET and
increase at higher operating frequencies and input
voltages. Transition losses can be estimated from:
Transition Loss = 2(V
IN
)
2
I
O(MAX)
C
RSS
(f)
Other losses including C
IN
and C
OUT
ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
Foldback Current Limiting
As described in the Output Diode Selection, the worst-
case dissipation occurs with a short-circuited output
when the diode conducts the current limit value almost
continuously. To prevent excessive heating in the diode,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding di-
odes D
FB1
and D
FB2
between the output and the I
TH
/RUN
pin as shown in Figure 6. In a hard short (V
OUT
= 0V), the
current will be reduced to approximately 50% of the
maximum output current.
Figure 5. Setting Output Voltage
3
V
FB
V
OUT
LTC1779
R1
1779 F05
R2
For most applications, an 80k resistor is suggested for R1.
To prevent stray pickup, locate resistors R1 and R2 close
to LTC1779.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (
η
1 +
η
2 +
η
3 + ...)
where
η
1,
η
2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1779 circuits: 1) LTC1779 DC bias current,
2) MOSFET gate charge current, 3) I
2
R losses and 4)
voltage drop of the output diode.
1. The V
IN
current is the DC supply current, given in the
electrical characteristics, that excludes MOSFET driver
and control currents. V
IN
current results in a small loss
which increases with V
IN
.
2. MOSFET gate charge current results from switching
the gate capacitance of the internal power MOSFET.
Each time the MOSFET gate is switched from low to
high to low again, a packet of charge dQ moves from
V
IN
to ground. The resulting dQ/dt is a current out of
V
IN
which is typically much larger than the DC supply
current. In continuous mode, I
GATECHG
= f(Qp).
3. I
2
R losses are predicted from the DC resistances of the
internal MOSFET, inductor and current shunt. In con-
tinuous mode the average output current flows through
L but is “chopped” between the internal P-channel
MOSFET in series with R
SENSE
and the output diode.
Figure 6. Foldback Current Limiting
V
FB
I
TH
/RUN
V
OUT
LTC1779
R1
1779 F06
R2
D
FB1
D
FB2
+
APPLICATIOU
W
U
U
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