LM27341_14 datasheet(19/42 Pages) TI1

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D =

VOUT + VD1 + VDCR

VIN + VD1 - VDS

LM27341, LM27342, LM27341Q, LM27342Q

www.ti.com

SNVS497D – NOVEMBER 2008 – REVISED FEBRUARY 2010

VDS is the voltage drop across the internal NFET when it is on, and is equal to:

VDS = IOUT x RDSON

(27)

VD is the forward voltage drop across the Schottky diode. It can be obtained from the Electrical Characteristics

section of the schottky diode datasheet. If the voltage drop across the inductor (VDCR) is accounted for, the

equation becomes:

(28)

VDCR usually gives only a minor duty cycle change, and has been omitted in the examples for simplicity.

SCHOTTKY DIODE CONDUCTION LOSSES

The conduction losses in the free-wheeling Schottky diode are calculated as follows:

PDIODE = VD1 x IOUT (1-D)

(29)

Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky

diode that has a low forward voltage drop.

INDUCTOR CONDUCTION LOSSES

Another significant external power loss is the conduction loss in the output inductor. The equation can be

simplified to:

PIND = IOUT

2 x R

DCR

(30)

MOSFET CONDUCTION LOSSES

The LM27341/LM27342 conduction loss is mainly associated with the internal NFET:

PCOND = IOUT

2 x R

DSON x D

(31)

MOSFET SWITCHING LOSSES

Switching losses are also associated with the internal NFET. They occur during the switch on and off transition

periods, where voltages and currents overlap resulting in power loss. The simplest means to determine this loss

is to empirically measuring the rise and fall times (10% to 90%) of the switch at the switch node:

PSWF = 1/2(VIN x IOUT x fSW x tFALL)

(32)

PSWR = 1/2(VIN x IOUT x fSW x tRISE)

(33)

PSW = PSWF + PSWR

(34)

Table 1. Typical Rise and Fall Times vs Input Voltage

VIN

tRISE

tFALL

8ns

10V

9ns

15V

10ns

IC QUIESCENT LOSSES

Another loss is the power required for operation of the internal circuitry:

PQ = IQ x VIN

(35)

IQ is the quiescent operating current, and is typically around 2.4 mA.

MOSFET DRIVER LOSSES

The other operating power that needs to be calculated is that required to drive the internal NFET:

PBOOST = IBOOST x VBOOST

(36)

VBOOST is normally between 3VDC and 5VDC. The IBOOST rms current is dependant on switching frequency fSW.

IBOOST is approximately 8.2 mA at 2 MHz and 4.4 mA at 1 MHz.

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