LTC1622 データシートの表示（PDF） - Linear Technology
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LTC1622 Datasheet PDF : 16 Pages
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LTC1622
APPLICATIONS INFORMATION
101
VREF
100
99
VITH
98
97
96
95
2.0
2.2
2.4
2.6
2.8
3.0
INPUT VOLTAGE (V)
1622 F03
Figure 3. Line Regulation of VREF and VITH
the maximum current sense voltage that sets the maxi-
mum output current.
Setting Output Voltage
The LTC1622 develops a 0.8V reference voltage between
the feedback (Pin 3) terminal and ground (see Figure 4). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the output voltage. The regulated
output voltage is determined by:
VOUT
=
0.81
+
R2
R1
For most applications, a 30k resistor is suggested for R1.
To prevent stray pickup, an optional 100pF capacitor is
suggested across R1 located close to LTC1622.
LTC1622
3
VFB
R2
100pF R1
VOUT
1622 F04
Figure 4. 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 LTC1622 circuits: 1) LTC1622 DC bias current,
2) MOSFET gate charge current, 3) I2R losses, 4) voltage
drop of the output diode and 5) transition losses.
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 power MOSFET. Each time
a 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
MOSFET, inductor and current shunt. In continuous
mode the average output current flows through L but
is “chopped” between the 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 resistance of the inductor 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 drop 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 external MOSFET and
increase with higher operating frequencies and input
voltages. Transition losses can be estimated from:
10
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