LTC3801 LINER [Linear Technology], LTC3801 Datasheet - Page 10

• Manufacturer Part Number: LTC3801
• Description: Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3801/LTC3801B
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:
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.
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:
where 1, 2, etc. are the individual losses as a percent-
age of input power.
10
Efficiency = 100% – ( 1 + 2 + 3 + ...)
V
OUT
0 8 1
.
Figure 4. Setting Output Voltage
LTC3801B
LTC3801/
U
R
V
R
FB
2
1
3
U
R2
R1
W
3801 F04
V
OUT
U
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
1. The V
2. MOSFET gate charge current results from switching the
3. I
4. The output diode is a major source of power loss at high
5. Transition losses apply to the external MOSFET and
Other losses including C
losses, and inductor core losses, generally account for
less than 2% total additional loss.
electrical characteristics, that excludes MOSFET driver
and control currents. V
which increases with V
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
resulting dQ/dt is a current out of V
much larger than the DC supply current. In continuous
mode, I
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
plus R
the resistances of L and R
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.
increase at higher operating frequencies and input
voltages. Transition losses can be estimated from:
Transition Loss = 2(V
2
R losses are predicted from the DC resistances of the
2
IN
SENSE
SENSE)
R losses and 4) voltage drop of the output diode.
GATECHG
current is the DC supply current, given in the
multiplied by duty cycle can be summed with
and the output diode. The MOSFET R
= (f)(dQ).
IN
IN
IN
)
IN
2
current results in a small loss
.
I
O(MAX)
SENSE
and C
to obtain I
C
OUT
RSS
IN
IN
which is typically
(f)
ESR dissipative
to ground. The
2
R losses.
DS(ON)
3801f