10

LTC3801/LTC3801B

sn3801 3801fs

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

R

R









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-

electrical characteristics, that excludes MOSFET driver

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,

much larger than the DC supply current. In continuous

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

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:

losses, and inductor core losses, generally account for

less than 2% total additional loss.

Figure 4. Setting Output Voltage

3

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.