10
LTC3801/LTC3801B
sn3801 3801fs
APPLICATIO S I FOR ATIO
WUUU
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS, AVX TPSV and KEMET T510 series of surface mount
tantalum, available in case heights ranging from 2mm to
4mm. Other capacitor types include Sanyo OS-CON,
Nichicon PL series and Panasonic SP.
Setting Output Voltage
The LTC3801/LTC3801B develop 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 overall output
voltage. The regulated output voltage is determined by:
V
R
R
OUT
=+
08 1
2
1
.
For most applications, an 80k resistor is suggested for R1.
In applications where low no-load quiescent current is
critical, R1 should be made >400k to limit the feedback
divider current to approximately 10% of the chip quiescent
current. If R2 then results in a very high impedance, it may
be beneficial to bypass R2 with a 5pF to 10pF capacitor. To
prevent stray pickup, locate resistors R1 and R2 close to
LTC3801/LTC3801B.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3801/LTC3801B circuits: 1) LTC3801/
LTC3801B DC bias current, 2) MOSFET gate charge cur-
rent, 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 power MOSFET. Each time a
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)(dQ).
3. I
2
R 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 R
SENSE)
and the output diode. 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 external 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.
Figure 4. Setting Output Voltage
3
V
FB
V
OUT
LTC3801/
LTC3801B
R1
3801 F04
R2
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.
