ZXLD1374_11 DIODES [Diodes Incorporated], ZXLD1374_11 Datasheet - Page 16

• Manufacturer Part Number: ZXLD1374_11
• Description: 60V HIGH ACCURACY 1.5A BUCK/BOOST/BUCK-BOOST
• Manufacturer: DIODES [Diodes Incorporated]

ZXLD1374
Document number: DS35032 Rev. 2 - 2
Applications Information
Component Selection
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 and the inductor/frequency setting. 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 R
GI pin, according to the device working mode. The sense resistor R
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
gain compensation factor, K, compensates for GI being 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 .
With V
pin determines the ratio of average LED current (I
resistor/coil current.
Where
Therefore:
I
COIL
I
LED
ADJ
=
I
I
=
LED
LED
LED
V
R
equal to V
1
V
I
LED
ADJ
−
s
) equal to the average sense resistor/coil current (I
GI
D
=
=
=
ADJ
V
V
(R
REF
ADJ
I
Rs
I
⇒
if the maximum input voltage is exceeded.
GI
Rs
R
1
REF
. 0
+
GI
=
R
=
225
1
R
K
, the ratio defined by the resistor divider at the GI
S
218
GI
V
225
2
R
RS
=
)
R
mV
S
225
mV
S
R =
=
I
s
=
LED
I
COIL
mV
V
V
(R
(Continued)
REF
ADJ
218
LED
xR
GI
I
LED
R
1
V
V
:
mV
+
=
REF
S
ADJ
GI
R
60V HIGH ACCURACY 1.5A BUCK/BOOST/BUCK-BOOST
218
1
=
REF
GI
R
I
LED
2
mV
S
1
)
, this becomes:
−
V
V
xR
D
REF
ADJ
V
V
REF
S
ADJ
LED
www.diodes.com
. 0
R
) to average sense
225
where K = 0.97
S
16 of 37
LED DRIVER CONVERTER WITH AEC-Q100
RS
). A loop
S
sets the coil current I
S
REF
, the voltage on the ADJ pin and the voltage on the
) to define the nominal 100% LED current. The
Diodes Incorporated
A Product Line of
Figure 31: Boost and Buck-boost
Figure 30: Buck configuration
RS
R
R
.
GI2
GI1
connection
REF
ADJ
REF
ADJ
SGND
SGND
GI
GI
VIN
VIN
R
R
S
S
ISM
ISM
© Diodes Incorporated
ZXLD1374
March 2011