LM3495_14 datasheet(26/35 Pages) TI1

No. de pieza	LM3495

Descripción Electrónicos	Emulated Peak Current Mode Buck Controller for Low Output Voltage

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Fabricante Electrónico	TI1 [Texas Instruments]

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K =

PO

PO + PTOTAL-LOSS

100

1k

10k

100k

1M

FREQUENCY (Hz)

-180

-160

-140

-120

-100

-80

-60

LM3495

SNVS410E – FEBRUARY 2006 – REVISED NOVEMBER 2007

www.ti.com

Figure 11. Overall Loop Gain and Phase

The bandwidth of this example circuit is 49 kHz, with a phase margin of 46°.

Efficiency Calculations

A reasonable estimation for the efficiency,

η, of a buck regulator controlled by the LM3495 can be obtained by

adding together the loss in each current carrying element, PTOTAL-LOSS, and using the equation:

(39)

The following shows an efficiency calculation to complement the Typical Application circuit. Output power for this

circuit is PO = 1.2V x 10A = 12W. Input voltage is assumed to be 12V, and the calculations used assume that the

converter runs in CCM.

Chip Operating Loss

This term accounts for the current drawn at the VIN pin. This current, IIN, drives the logic circuitry and the power

FETs. The gate driving loss term from the power FET section of Design Considerations is included in the chip

operating loss. For the LM3495, IIN is equal to the steady state operating current, IQ, plus the FET driving current,

IGC. Power is lost as this IIN passes through the internal linear regulator of the LM3495.

IGC = (QG-HI + QG-LO) x fOSC

(40)

IGC = (11nC + 33nC) x 500 kHz = 22 mA

(41)

IQ is typically 1.8 mA, taken from the Electrical Characteristics table. Chip Operating Loss is then:

PQ = VIN x (IQ + IGC)

(42)

PQ = 12V x (1.8mA + 22mA) = 0.29W

(43)

High-Side FET Switching Loss

PSW = 0.5 x VIN x IO x (tR + tF) x fSW

(44)

PSW = 0.5 x 12V x 10A x (5 ns + 8 ns) x 500 kHz = 0.39W

(45)

FET Conduction Loss

PC = D (I

2

O x RDSON-HI x 1.3) PC-HI = 0.1 x (100 x 0.013) = 0.13W

(46)

PC = (1 - D) (I

2

O x RDSON-LO x 1.3) PC-LO = 0.9 x (100 x 0.0044) = 0.40W

(47)

RSNS Loss (if used)

PSNS = (1 - D) ((IO)

2 x R

SNS)

(48)

Not used in this example.

Input Capacitor Loss

26

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