Application information, A product line of diodes incorporated – Diodes ZXLD1356/ ZXLD1356Q User Manual

Page 24

Advertising
background image

ZXLD1356/ ZXLD1356Q

Document number: DS33470 Rev. 4 - 2

24 of 29

www.diodes.com

October 2012

© Diodes Incorporated

ZXLD1356/ ZXLD1356Q

A Product Line of

Diodes Incorporated


Application Information

(cont.)

Reducing Output Ripple

Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor, Cled, across the LED(s) as shown below:

V

IN

V

IN

I

SENSE

LX

ZXLD1356

Rs

L1

Cled

LED

D1

A value of 1µF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor

values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of

LED voltage.

By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version without altering

the mean current value.

Operation at Low Supply Voltage

Below the under-voltage lockout threshold (V

SD

) the drive to the output transistor is turned off to prevent device operation with excessive on-

resistance of the output transistor. The output transistor is not full enhanced until the supply voltage exceeds approximately 17V. At supply

voltages between V

SD

and 17V care must be taken to avoid excessive power dissipation due to the on-resistance.

Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient to prevent the device from switching below

approximately 6V. This will minimize the risk of damage to the device.

Thermal Considerations

When operating the device at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the

package power dissipation limits. The graph below gives details for power derating. This assumes the device to be mounted on a (25mm)

2

PCB

with 1oz copper standing in still air.

TSOT23-5

DFN3030-6

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-40

-25

-10

5

20

35

50

65

80

95

110

125

Ambient Temperature (°C)

140

Maximum Power Dissipation

P

o

we

r Dis

s

ip

a

tio

n

(W

)

Advertising