ZXLD1370

ZXLD1370

Document number: DS32165 Rev. 3 - 2

21 of 35

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February 2011

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Assuming that cumulatively the rise time and fall time can account for a maximum of 10% of the period, the maximum

frequency allowed in this condition is:

This frequency is within the recommended frequency range the device can handle, therefore the ZXMN6A09K is

recommended to be used with the ZXLD1370 for frequencies from 300kHz to 500kHz).

The recommended total gate charge for the MOSFET used in conjunction with the ZXLD1370 is less than 30nC.

Junction temperature estimation

Finally, the ZXLD1370 junction temperature can be estimated using the following equations:

Total supply current of ZXLD1370:

Power consumed by ZXLD1370

discharge the gate of the power MOSFET. Moreover the part of thermal resistance between case and ambient depends on

the PCB characteristics.

DIODE SELECTION

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode* with low

reverse leakage at the maximum operating voltage and temperature. The Schottky diode also provides better efficiency

than silicon PN diodes, due to a combination of lower forward voltage and reduced recovery time.

It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher

than the maximum output load current. In particular, it is recommended to have a voltage rating at least 15% higher than

the maximum transistor voltage to ensure safe operation during the ringing of the switch node and a current rating at least

10% higher than the average diode current. The power rating is verified by calculating the power loss through the diode.

The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on

the Drain of the external MOSFET. If a silicon diode is used, care should be taken to ensure that the total voltage appearing

on the Drain of the external MOSFET, including supply ripple, does not exceed the specified maximum value.

*A suitable Schottky diode would be PDS3100 (Diodes Inc).

OUTPUT CAPACITOR

An output capacitor may be required to limit interference or for specific EMC purposes.

For boost and buck-boost

regulators, the output capacitor provides energy to the load when the freewheeling diode is reverse biased during the first

switching subinterval. An output capacitor in a buck topology will simply reduce the LED current ripple below the inductor

current ripple. In other words, this capacitor changes the current waveform through the LED(s) from a triangular ramp to a

more sinusoidal version without altering the mean current value.

In all cases, the output capacitor is chosen to provide a desired current ripple of the LED current (usually recommended to

be less than 40% of the average LED current).

Buck:

PP

LED

LED

SW

PP

L

OUTPUT

I

x

r

x

f

x

8

I

C

−

−

Δ

Δ

=