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LTC1772

1772fb

Low Supply Operation

Although the LTC1772 can function down to approxi-

mately 2V, the maximum allowable output current is

reduced when VIN decreases below 3V. Figure 3 shows the

amount of change as the supply is reduced down to 2V.

Also shown in Figure 3 is the effect of VIN on VREF as VIN

goes below 2.3V.

APPLICATIONS INFORMATION

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.

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 se-

ries 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

Figure 4. Setting Output Voltage

VFB

VOUT

LTC1772

1772 F04

Figure 3. Line Regulation of VREF and VITH

INPUT VOLTAGE (V)

2.0

105

100

2.2

2.4

2.6

2.8

1772 F03

3.0

VREF

VITH

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, locate resistors R1 and R2 close

to LTC1772.

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