LT3758 datasheet(13/20 Pages) LINER

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LTC3872-1

38721f

For more information www.linear.com/LTC3872-1

Input Capacitor Selection

The input capacitor of a boost converter is less critical

than the output capacitor, due to the fact that the inductor

is in series with the input and the input current waveform

is continuous (see Figure 6b). The input voltage source

impedance determines the size of the input capacitor,

which is typically in the range of 10µF to 100µF. A low ESR

capacitor is recommended, although it is not as critical as

for the output capacitor.

The RMS input capacitor ripple current for a boost con-

verter is:

IRMS(CIN) =0.3•

VIN(MIN)

L • f

•DMAX

Please note that the input capacitor can see a very high

surge current when a battery is suddenly connected to

the input of the converter and solid tantalum capacitors

can fail catastrophically under these conditions. Be sure

to specify surge-tested capacitors!

Efficiency Considerations: How Much Does VDS

Sensing Help?

Theefficiencyofaswitchingregulatorisequaltotheoutput

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

individuallossestodeterminewhatislimitingtheefficiency

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 LTC3872-1 application circuits:

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

sum of the DC supply current IQ (given in the Electrical

Characteristics) and the MOSFET driver and control cur-

rents. The DC supply current into the VIN pin is typically

about 250µA 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 VIN

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

supplied to the Input capacitor 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. The

technique of using the voltage drop across the power

MOSFET to close the current feedback loop was chosen

because of the increased efficiency that results from not

having a sense resistor. The losses in the power MOSFET

are equal to:

PFET =

IO(MAX)

1– DMAX









•RDS(ON) •DMAX • ρT

+ k • VO 1.85 •

IO(MAX)

1– DMAX

• CRSS • f

TheI2Rpowersavingsthatresultfromnothavingadiscrete

sense resistor can be calculated almost by inspection.

PR(SENSE) =

IO(MAX)

1– DMAX









•RSENSE •DMAX

To understand the magnitude of the improvement with

this VDS sensing technique, consider the 3.3V input, 5V

output power supply shown in the Typical Application on

thefrontpage.Themaximumloadcurrentis7A(10Apeak)

and the duty cycle is 39%. Assuming a ripple current of

40%, the peak inductor current is 13.8A and the average

applicaTions inForMaTion

Figure 7. Load Transient Response for a 3.3V Input,

5V Output Boost Converter Application, 0.1A to 1A Step

ILOAD

500mA/DIV

VOUT

200mV/DIV

AC-COUPLED

20µs/DIV

38721 F07

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