LTC1772
U
W U U
APPLICATIONS INFORMATION
Setting Output Voltage
1. The VIN current is the DC supply current, given in the
electricalcharacteristics, thatexcludesMOSFETdriver
and control currents. VIN current results in a small loss
which increases with VIN.
The LTC1772 develops a 0.8V reference voltage between
thefeedback(Pin3)terminalandground(seeFigure4).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:
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,apacketofchargedQmovesfromVIN toground.
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
withRSENSE andtheoutputdiode.TheMOSFETRDS(ON)
plus RSENSE multiplied by duty cycle can be summed
with the resistances of L and RSENSE to obtain I2R
losses.
R2
VOUT = 0.8 1+
R1
Formostapplications, an80kresistorissuggestedforR1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to LTC1772.
V
OUT
R2
R1
LTC1772
3
V
FB
100pF
1772 F04
Figure 4. Setting Output Voltage
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.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
oftenusefultoanalyzeindividuallossestodeterminewhat
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
5. Transition losses apply to the external MOSFET and
increase at higher operating frequencies and input
voltages. Transition losses can be estimated from:
Efficiency = 100% – (η1 + η2 + η3 + ...)
where η1, η2, etc. are the individual losses as a percent-
age of input power.
Transition Loss = 2(VIN)2IO(MAX) RSS
(f)
C
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1772 circuits: 1) LTC1772 DC bias current,
2) MOSFET gate charge current, 3) I2R losses and 4)
voltage drop of the output diode.
Other losses including CIN and COUT ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
9
Linear [ Linear ]
