LTC3801B_15 Datasheet, PDF (10/12 Pages) Linear Technology – Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT

LTC3801/LTC3801B
APPLICATIO S I FOR ATIO
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_OUT = 0.8^{}1+ ^{RR21}
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.
LTC3801/
LTC3801B
3
V_FB
V_OUT
R2
R1
3801 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.
10
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) I²R 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. 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 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 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 external MOSFET and
increase at higher operating frequencies and input
voltages. Transition losses can be estimated from:
Transition Loss = 2(V_IN)²I_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.
sn3801 3801fs