AAT4900IJS-T1 datasheet(8/13 Pages) AAT

Part #	AAT4900IJS-T1

Description	Buffered Power Half-Bridge

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Manufacturer	AAT [Advanced Analog Technology, Inc.]

Direct Link	http://www.aatech.com.tw

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AAT4900

Buffered Power Half-Bridge

8

4900.2006.05.1.3

Synchronous Buck DC/DC Converter

Application

The losses associated with the AAT4900 high side

switching MOSFET are due to switching losses and

conduction losses. The conduction losses are asso-

ciated with the RDS(ON) characteristics of the output

switching device. At the full load condition, assum-

ing continuous conduction mode (CCM), the on

losses can be derived from the following equations.

Eq. 1:

D is the duty cycle.

Eq. 2:

ΔI is the peak-to-peak inductor ripple current.

High Side Switch RMS Current

Eq. 3:

Low Side Switch RMS Current

The low side RMS current is estimated by the fol-

lowing equation.

Eq. 4:

Total Losses

A simplified form of the above results (where the

above descriptions of IRMS has been approximated

with Io) is given by:

Eq. 5:

Substitution of the IRMS equations with IO results in

very little error when the inductor ripple current is

20% to 40% of the full load current. The equation

also includes switching and quiescent current loss-

es where tSW is approximated at 18 nsec and IQ is

the no load quiescent current of the AAT4900.

Quiescent current losses are associated with the

gate drive of the output stage and biasing. Since

the gate drive current varies with frequency and

voltage, the bias current must be checked at the

frequency, voltage, and temperature of operation

with no load attached to the LX node. Once the

above losses have been determined, the maximum

junction temperature can be calculated.

Eq. 6:

Using the above equations, the graph below shows

the current capability for some typical applications

with maximum junction temperatures of 150°C and

120°C. The increase in RDS(ON) vs. temperature is

estimated at 3.75m

Ω for a 10°C increase in junc-

tion temperature.

Step-Down Converter Limits

(FS = 1MHz)

0.5

0.75

1

1.25

1.5

1.75

25

35

45

55

65

75

85

Ambient Temperature (

°C)

TJMAX = 150

°C

VIN = 4.2V, VO = 2.5V

VIN = 5.0V, VO = 3.3V

VIN = 4.2V, VO = 2.5V

VIN = 5.0V, VO = 3.3V

TJMAX = 120

°C

T

J(MAX) = PLOSS ·

ΘJC = TAMB

P

LOSS

+ (t

sw · FS · IO + IQ) · VIN

I

O

2 · (R

DS(ON)H · VO + RDS(ON)L · (VIN -VO))

V

IN

=

2

12

O

I

⎛⎞

Δ

=

+

· (1 - D)

⎝⎠

IRMS(LS)

2

12

O

I

⎛⎞

Δ

=

+

· D

⎝⎠

IRMS(HS)

1

ΔI =

⎛⎞

-

⎝⎠

VO

VIN

VO

L · FS

D = O

IN

V

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