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LTC1871-7

18717fc

APPLICATIONS INFORMATION

During this inductor charging interval, the output capacitor

must supply the load current and a signiﬁcant droop in

the output voltage can occur. Generally, it is a good idea

to choose a value of inductor ΔIL between 25% and 40%

of IIN(MAX). The alternative is to either increase the value

of the output capacitor or disable Burst Mode operation

using the MODE/SYNC pin.

Burst Mode operation can be defeated by connecting the

MODE/SYNC pin to a high logic-level voltage (either with

a control input or by connecting this pin to INTVCC). In

this mode, the burst clamp is removed, and the chip can

operate at constant frequency from continuous conduction

mode (CCM) at full load, down into deep discontinuous

conduction mode (DCM) at light load. Prior to skipping

pulses at very light load (i.e., <5% of full load), the control-

ler will operate with a minimum switch on-time in DCM.

Pulse skipping prevents a loss of control of the output at

very light loads and reduces output voltage ripple.

Efﬁciency Considerations

The efﬁciency of a switching regulator is equal to the out-

put power divided by the input power (¥100%). Percent

efﬁciency can be expressed as:

% Efﬁciency = 100% – (L1 + L2 + L3 + …),

where L1, L2, etc. are the individual loss components as a

percentage of the input power. It is often useful to analyze

individual losses to determine what is limiting the efﬁciency

and which change would produce the most improvement.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for the majority

of the losses in LTC1871-7 application circuits:

1. The supply current into VIN. The VIN current is the sum

of the DC supply current IQ (given in the Electrical Char-

acteristics) and the MOSFET driver and control currents.

The DC supply current into the VIN pin is typically about

650μA and represents a small power loss (much less

than 1%) that increases with VIN. The driver current

results from switching the gate capacitance of the power

MOSFET; this current is typically much larger than the

DC current. Each time the MOSFET is switched on and

then off, a packet of gate charge QG is transferred from

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

must be supplied to the INTVCC capacitor through the

VIN pin by an external supply. If the IC is operating in

CCM:

IQ(TOT) ≈ IQ = f • QG

PIC = VIN • (IQ + f • QG)

2. Power MOSFET switching and conduction losses:

PFET =

IO(MAX)

1–DMAX

•RDS(ON) •DMAX • T

+ k• VO2 •

IO(MAX)

1–DMAX

•CRSS •f

3. The I2R losses in the sense resistor can be calculated

almost by inspection.

PR(SENSE) =

IO(MAX)

1–DMAX

•RSENSE •DMAX

4. The losses in the inductor are simply the DC input cur-

rent squared times the winding resistance. Expressing

this loss as a function of the output current yields:

PR(WINDING) =

IO(MAX)

1–DMAX

•RW

5. Losses in the boost diode. The power dissipation in the

boost diode is:

PDIODE = IO(MAX) • VD

The boost diode can be a major source of power loss

in a boost converter. For 13.2V input, 42V output at

1.5A example given in Figure 9, a Schottky diode with

a 0.4V forward voltage would dissipate 600mW, which

represents about 1% of the input power. Diode losses

can become signiﬁcant at low output voltages where

the forward voltage is a signiﬁcant percentage of the

output voltage.

6. Other losses, including CIN and CO ESR dissipation and

inductor core losses, generally account for less than

2% of the total losses.

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