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LTC7813

7813f

For more information www.linear.com/LTC7813

applicaTions inForMaTion

The MOSFET power dissipations at maximum output

current are given by:

PMAIN_BUCK =

VOUT

VIN

IOUT(MAX)

(

)

2 1+δ

(

)RDS(ON) +

(VIN)2

IOUT(MAX)

⎛

⎝

⎜

⎞

⎠

⎟(RDR)(CMILLER) •

VDRVCC − VTHMIN

VTHMIN

⎡

⎣

⎢

⎤

⎦

⎥(f)

PSYNC_BUCK =

VIN − VOUT

VIN

IOUT(MAX)

(

)

2 1+δ

(

)RDS(ON)

PMAIN_BOOST =

VOUT − VIN

(

)VOUT

VIN2

IOUT(MAX)

(

)

2 •

1+δ

(

)RDS(ON) +

VOUT3

VIN

⎛

⎝

⎜

⎞

⎠

⎟

IOUT(MAX)

⎛

⎝

⎜

⎞

⎠

⎟ •

RDR

(

) CMILLER

(

) •

VDRVCC − VTHMIN

VTHMIN

⎡

⎣

⎢

⎤

⎦

⎥(f)

PSYNC_BOOST =

VIN

VOUT

IOUT(MAX)

(

)

2 1+δ

(

)RDS(ON)

where δ is the temperature dependency of RDS(ON) and

RDR (approximately 2Ω) is the effective driver resistance

at the MOSFET’s Miller threshold voltage. VTHMIN is the

typical MOSFET minimum threshold voltage.

Both MOSFETs have I2R losses while the main N-channel

equations for the buck and boost controllers include an

additional term for transition losses, which are highest at

high input voltages for the buck and low input voltages

for the boost. For VIN < 20V (higher VIN for the boost)

the high current efficiency generally improves with larger

MOSFETs, while for VIN > 20V (lower VIN for the boost)

the transition losses rapidly increase to the point that the

use of a higher RDS(ON) device with lower CMILLER actu-

ally provides higher efficiency. The synchronous MOSFET

losses for the buck controller are greatest at high input

voltage when the top switch duty factor is low or during

a short-circuit when the synchronous switch is on close

to 100% of the period.

The term (1 + δ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs Temperature curve, but

δ = 0.005/°C can be used as an approximation for low

voltage MOSFETs.

Boost CIN, COUT Selection

The input ripple current in a boost converter is relatively

low (compared with the output ripple current), because

this current is continuous. The boost input capacitor CIN

voltage rating should comfortably exceed the maximum

inputvoltage.Althoughceramiccapacitorscanberelatively

tolerant of overvoltage conditions, aluminum electrolytic

capacitorsarenot.Besuretocharacterizetheinputvoltage

for any possible overvoltage transients that could apply

excess stress to the input capacitors.

The value of CIN is a function of the source impedance, and

in general, the higher the source impedance, the higher the

required input capacitance. The required amount of input

capacitance is also greatly affected by the duty cycle. High

output current applications that also experience high duty

cycles can place great demands on the input supply, both

in terms of DC current and ripple current.

Inaboostconverter,theoutputhasadiscontinuouscurrent,

so COUT must be capable of reducing the output voltage

ripple. The effects of ESR (equivalent series resistance)

and the bulk capacitance must be considered when choos-

ing the right capacitor for a given output ripple voltage.

The steady ripple due to charging and discharging the

bulk capacitance is given by:

Ripple =

IOUT(MAX) • VOUT − VIN(MIN)

(

)

COUT • VOUT • f

where COUT is the output filter capacitor.

The steady ripple due to the voltage drop across the ESR

is given by:

∆VESR = IL(MAX) • ESR

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic

and ceramic capacitors are all available in surface mount

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