744770933 データシートの表示（PDF） - Diodes Incorporated.
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744770933 Datasheet PDF : 33 Pages
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A Product Line of
Diodes Incorporated
ZXLD1370
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 IRS.
The ADJ pin may be connected directly to the internal 1.25V reference (VREF) to define the nominal 100% LED current. The
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 VADJ if the maximum input voltage is exceeded.
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 giving the ratio of
RS
average LED current (ILED) to average sense resistor/coil
current (IRS).
VIN
ISM
ILED =
IRs
= 225mV VADJ
R S VREF
REF
ADJ
If the ADJ and GI pins are connected to VREF directly, this
becomes:
ILED =
IRs
= 225mV
RS
Therefore:
Rs =
225mV
ILED
GI
SGND
Figure 17: Buck configuration
In Boost and Buck-boost mode GI is connected to ADJ
through a voltage divider.
With VADJ equal to VREF, the ratio defined by the resistor
divider at the GI pin determines the ratio of average LED
current (ILED) to average sense resistor/coil current (IRS).
ILED
=
VGI
VADJ
IRs
=
(R
R GI1
GI1 + R
GI2
)
IRs
Where
IRs
=
225mV VADJ
RS VREF
When the ADJ pin is connected to VREF directly, this
becomes:
IRs
=
225mV
RS
RS
VIN
ISM
REF
R GI2
R GI1
ADJ
GI
SGND
Figure 18: Boost and Buck-boost connection
ZXLD1370
Document number: DS32165 Rev. 2 - 2
12 of 33
www.diodes.com
May 2010
© Diodes Incorporated
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