LTC1622CS8 datasheet(10/16 Pages) LINER

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LTC1622

the maximum current sense voltage that sets the maxi-

mum output current.

Setting Output Voltage

The LTC1622 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 output voltage. The regulated

output voltage is determined by:

OUT =+









08 1

For most applications, a 30k resistor is suggested for R1.

To prevent stray pickup, an optional 100pF capacitor is

suggested across R1 located close to LTC1622.

APPLICATIONS INFORMATION

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 LTC1622 circuits: 1) LTC1622 DC bias current,

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

drop of the output diode and 5) transition losses.

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 resistance of the inductor 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 drop 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 with higher operating frequencies and input

voltages. Transition losses can be estimated from:

VFB

VOUT

LTC1622

100pF

1622 F04

Figure 4. Setting Output Voltage

INPUT VOLTAGE (V)

2.0

101

100

2.2

2.4

2.6

2.8

1622 F03

3.0

VREF

VITH

Figure 3. Line Regulation of VREF and VITH

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

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