ZXLD1370_11 DIODES [Diodes Incorporated], ZXLD1370_11 Datasheet - Page 21

• Manufacturer Part Number: ZXLD1370_11
• Description: 60V HIGH ACCURACY BUCK/BOOST/BUCK-BOOST LED DRIVER CONTROLLER
• Manufacturer: DIODES [Diodes Incorporated]

Or in case of separate voltage supply, with V
ZXLD1370
Document number: DS32165 Rev. 3 - 2
Applications Information
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
Where the total quiescent current IQ
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.
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:
t
I
Where I
P
P
T
C
PERIOD
QTOT
J
IC
IC
OUTPUT
=
= V
= V
≈ I
IN
IN
= 20*dt
Q
Q
T
T
=
• (I
• I
= total quiescent current I
A
A
+ f • Q
8
Q-IN
+ P
+ P
Q
f x
+ f • Qg)
SW
IC
IC
+ V
G
• R
• (R
r x
aux
Δ
TH(JA)
LED
I
TH(JC)
L
• (I
−
PP
x
Q-AUX
Δ
=
f = 1/ t
+ R
I
(Continued)
LED
TOT
TH(CA))
−
+ f • Qg)
PP
PERIOD
consists of the static supply current (IQ) and the current required to charge and
AUX
Q-IN
< 15V
+ I
= 515kHz
Q-AUX
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ZXLD1370
February 2011