TJ4519DP 参数 Datasheet PDF下载

600kHz 3A Step-Down Switching Regulator

C

IN

= I

OMAX

•

D • (1 - D)

f

s

• (

Δ

V

I

I

OMAX

• R

CIN(ESR )

)

TJ4519

Where:

∆

V

I

= the given input voltage ripple.

Because the input capacitor is exposed to the large surge current, attention is needed for the input capacitor. If

tantalum capacitors are used at the input side of the converter, one needs to ensure that the RMS and surge

ratings are not exceeded. For generic tantalum capacitors, it is suggested to derate their voltage ratings at a ratio

of about two to protect these input capacitors.

Boost Capacitor and its Supply Source Selection

The boost capacitor selection is based on its discharge ripple voltage, worst case conduction time and boost

current. The worst case conduction time and boost current. The worst case conduction time T

W

can be estimated

as follows:

T

W

=

1

• D

max

f

s

Where:

f

S

= the switching frequency,

D

max

= maximum duty ratio, 0.85 for the TJ4915.

The required minimum capacitance for the boost capacitor will be:

C

boost

=

I

B

•T

V

D W

Where:

I

B

= the boost current and

V

D

= discharge ripple voltage.

With f

S

= 600kHz, V

D

= 0.5V and I

B

=0.045A, the required minimum capacitance for the boost capacitor is:

C

boost

=

I

B

1

0.045

1

• • D

max

=

•

• 0.85 = 128nF

V

D

f

s

0.5

600k

The internal driver of the switch requires a minimum 2.7V to fully turn on that switch to reduce its conduction

loss. If the output voltage is less than 2.7V, the boost capacitor can be connected to either the input side or an

independent supply with a decoupling capacitor. But the Pin BST should not see a voltage higher than its

maximum rating.

Freewheeling Diode Selection

This diode conducts during the switch’s off-time. The diode should have enough current capability for full load

and short circuit conditions without any thermal concerns. Its maximum repetitive reverse block voltage has to be

higher than the input voltage of the TJ4519. A low forward conduction drop is also required to increase the overall

efficiency. The freewheeling diode should be turned on and off fast with minimum reverse recovery because the

TJ4519 is designed for high frequency applications. SS23 Schottky rectifier is recommended for certain

applications. The average current of the diode, I

D-AVG

can be calculated by:

Jul. 2010 – Preliminary

-8-

HTC