LTC1772CS6 datasheet(9/16 Pages) LINER

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LTC1772

Setting Output Voltage

The LTC1772 develops 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:

OUT =+









08 1

For most applications, an 80k resistor is suggested for R1.

To prevent stray pickup, a 100pF capacitor is suggested

across R1 located close to LTC1772.

APPLICATIONS INFORMATION

1. The VIN current is the DC supply current, given in the

electrical characteristics, that excludes MOSFET driver

and control currents. VIN current results in a small loss

which increases with VIN.

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 VIN to ground.

The resulting dQ/dt is a current out of VIN which is

typically much larger than the DC supply current. In

continuous mode, IGATECHG = f(Qp).

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 RSENSE and the output diode. The MOSFET RDS(ON)

plus RSENSE multiplied by duty cycle can be summed

with the resistances of L and RSENSE 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(VIN)2IO(MAX)CRSS(f)

Other losses including CIN and COUT ESR dissipative

losses, and inductor core losses, generally account for

less than 2% total additional loss.

Figure 4. Setting Output Voltage

VFB

VOUT

LTC1772

100pF

1772 F04

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.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1772 circuits: 1) LTC1772 DC bias current,

2) MOSFET gate charge current, 3) I2R losses and 4)

voltage drop of the output diode.

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