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LTC1876

1876fa

Voltage Positioning

Voltage positioning can be used to minimize peak-to-peak

output voltage excursions under worst-case transient

loading conditions. The open loop DC gain of the control

loop is reduced depending upon the maximum load step

specifications. Voltage positioning can be easily added to

the LTC1876 by loading the ITH pin with a resistive divider

having a Thevenin equivalent voltage source equal to the

midpoint operating voltage of the error amplifier, or 1.2V

(see Figure 8).

The resistive load reduces the DC loop gain while main-

taining the linear control range of the error amplifier. The

maximum output voltage deviation can theoretically be

reduced to half or alternatively the amount of output

capacitance can be reduced for a particular application. A

complete explanation is included in Design Solutions 10.

(See: www.linear-tech.com)

1. The VIN current has two components: the first is the DC

supply current given in the Electrical Characteristics table,

which excludes MOSFET driver and control currents; the

second is the current drawn from the 3.3V linear regulator

output. VIN current typically results in a small (<0.1%)

loss.

2. INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results from

switching the gate capacitance of the power MOSFETs.

Each time a MOSFET gate is switched from low to high to

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

ground. The resulting dQ/dt is a current out of INTVCC that

is typically much larger than the control circuit current. In

continuous mode, IGATECHG =f(QT+QB), where QT and QB

are the gate charges of the topside and bottom side

MOSFETs.

Supplying INTVCC power through the EXTVCC switch input

from an output-derived source will scale the VIN current

required for the driver and control circuits by a factor of

(Duty Cycle)/(Efficiency). For example, in a 20V to 5V

application, 10mA of INTVCC current results in approxi-

mately 3mA of VIN current. This reduces the mid-current

loss from 10% or more (if the driver was powered directly

from VIN) to only a few percent.

3. I2R losses are predicted from the DC resistances of the

fuse (if used), MOSFET, inductor, current sense resistor,

and input and output capacitor ESR. In continuous mode

the average output current flows through L and RSENSE,

but is “chopped” between the topside MOSFET and the

synchronous MOSFET. If the two MOSFETs have approxi-

mately the same RDS(ON), then the resistance of one

MOSFET can simply be summed with the resistances of L,

RSENSE and ESR to obtain I2R losses. For example, if each

RDS(ON) = 30mΩ, RL = 50mΩ, RSENSE = 10mΩ and RESR

= 40m

Ω (sum of both input and output capacitance

losses), then the total resistance is 130m

Ω. This results in

losses ranging from 3% to 13% as the output current

increases from 1A to 5A for a 5V output, or a 4% to 20%

loss for a 3.3V output. Efficiency varies as the inverse

square of VOUT for the same external components and

output power level. The combined effects of increasingly

lower output voltages and higher currents required by

high performance digital systems is not doubling but

APPLICATIO S I FOR ATIO

ITH

RT1

INTVCC

1876 F08

LTC1876

RT2

Efficiency Considerations

The percent 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. Percent efficiency can be

expressed as:

%Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1876 circuits: 1) LTC1876 VIN current (in-

cluding loading on the 3.3V internal regulator), 2) INTVCC

regulator current, 3) I2R losses, 4) topside MOSFET

transition losses.

Figure 8. Active Voltage Positioning Applied to the LTC1876

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