ZXLD1370_11 DIODES [Diodes Incorporated], ZXLD1370_11 Datasheet - Page 13

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

Application Information
A basic ZXLD1370 application circuit is shown in Figure 13 and 15.
External component selection is driven by the characteristics of the load and the input supply, since this will determine the
kind of topology being used for the system.
Component selection starts with the current setting procedure, the inductor/frequency setting and the MOSFET selection.
Finally after selecting the freewheeling diode and the output capacitor (if needed), the application section will cover the PWM
dimming and thermal feedback.
Setting the output current
The first choice when defining the output current is whether the device is operating with the load in series with the sense
resistor (buck mode) or whether the load is not in series with the sense resistor (boost and buck-boost modes).
The output current setting depends on the choice of the sense resistor Rs, the voltage on the ADJ pin and the voltage on the
GI pin, according to the device working mode. The sense resistor Rs sets the coil current I
The ADJ pin may be connected directly to the internal 1.25V reference (V
ADJ pin can also be overdriven with an external dc voltage between 125mV and 2.5V to adjust the LED current proportionally
between 10% and 200% of the nominal value.
ADJ and GI are high impedance inputs within their normal operating voltage ranges. An internal 2.6V clamp protects the
device against excessive input voltage and limits the maximum output current to approximately 4% above the maximum
current set by V
Below are provided the details of the LED current calculation both when the load in series with the sense resistor (buck mode)
and when the load is not in series with the sense resistor (boost and buck-boost modes).
In Buck mode, GI is connected to ADJ which results in the average
LED current (I
(I
connected to ADJ. This gives the following equation for I
If ADJ (and GI pin) is directly connected to V
Therefore:
In Boost and Buck-boost mode GI is connected to ADJ through a voltage divider .
The ratio of average LED current (I
current is determined by voltage divider ratio (defined by the resistor
divider) at the GI pin.
Now
Rearranging gives
ZXLD1370
Document number: DS32165 Rev. 3 - 2
RS
). A loop gain compensation factor, K, compensates for GI being
I
I
I
R =
I
R
V
LED
LED
LED
COIL
RS
s
s
=
LED
=
218
=
=
(R
ADJ
I
I
=
COIL
) equal to the average sense resistor/coil current
LED
=
GI
:
=
mV
if the maximum input voltage is exceeded.
1
R
1
I
I
LED
V
(R
I
Rs
Rs
−
×
V
+
GI
ADJ
R
GI
D
R
GI
1
=
S
R
1
GI
=
=
+
K
GI
V
2
=
V
218
218
)
REF
R
⇒
1
ADJ
225
I
R
225
R
LED
GI
R
I
mV
S
S
1
mV
LED
2
S
mV
−
225
)
mV
×
D
V
Standard boost converter equation
V
(Continued)
R
R
LED
V
V
REF
ADJ
mV
S
V
V
S
REF
ADJ
V
V
) to average sense resistor/coil
REF
ADJ
REF
ADJ
225
=
R
=
mV
S
REF
Sense resistor voltage
, this becomes:
where K= 0.97
www.diodes.com
LED
:
13 of 35
REF
Figure 18. Boost and Buck-boost connection
) to define the nominal 100% LED current. The
Diodes Incorporated
A Product Line of
Figure 17. Buck configuration
R
R
RS
GI2
GI1
.
REF
ADJ
SGND
REF
ADJ
GI
SGND
VIN
GI
VIN
R
R
S
S
ISM
ISM
© Diodes Incorporated
ZXLD1370
February 2011