9

LTC1779

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1779 circuits: 1) LTC1779 DC bias current,

voltage drop of the output diode.

electrical characteristics, that excludes MOSFET driver

2. MOSFET gate charge current results from switching

the gate capacitance of the internal power MOSFET.

Each time the MOSFET gate is switched from low to

high to low again, a packet of charge dQ moves from

internal MOSFET, inductor and current shunt. In con-

tinuous mode the average output current flows through

L but is “chopped” between the internal P-channel

cycle can be summed with the resistances of L and

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 internal 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 5. Setting Output Voltage

3

LTC1779

R1

1779 F05

R2

Setting Output Voltage

The LTC1779 develops a 0.8V reference voltage between

the feedback (Pin 3) terminal and ground (see Figure 5). 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.

To prevent stray pickup, locate resistors R1 and R2 close

to LTC1779.

APPLICATIO S I FOR ATIO

INPUT VOLTAGE (V)

2.0

105

100

95

90

85

80

75

2.2

2.4

2.6

2.8

1779 F04

3.0

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.