LTC1779 データシートの表示（PDF） - Linear Technology
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LTC1779 Datasheet PDF : 12 Pages
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LTC1779
APPLICATIO S I FOR ATIO
105
VREF
100
95
VITH
90
85
80
75
2.0
2.2
2.4
2.6
2.8
3.0
INPUT VOLTAGE (V)
1779 F04
Figure 4. Line Regulation of VREF and VITH
Setting Output Voltage
The LTC1779 develops a 0.8V reference voltage between
the feedback (Pin 3) terminal and ground (see Figure 5). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
VOUT
=
0.8 1 +
R2
R1
For most applications, an 80k resistor is suggested for R1.
To prevent stray pickup, locate resistors R1 and R2 close
to LTC1779.
LTC1779
3
VFB
VOUT
R2
R1
1779 F05
Figure 5. Setting Output Voltage
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% – (η1 + η2 + η3 + ...)
where η1, η2, 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) I2R losses and 4)
voltage drop of the output diode.
1. The VIN current is the DC supply current, given in the
electrical characteristics, that excludes MOSFET driver
and control currents. VIN current results in a small loss
which increases with VIN.
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
VIN to ground. The resulting dQ/dt is a current out of
VIN which is typically much larger than the DC supply
current. In continuous mode, IGATECHG = f(Qp).
3. I2R 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 RSENSE and the output diode.
The MOSFET RDS(ON) plus RSENSE multiplied by duty
cycle can be summed with the resistances of L and
RSENSE to obtain I2R 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(VIN)2IO(MAX)CRSS(f)
Other losses including CIN and COUT ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
9
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