ZXLD1370

ZXLD1370

Document number: DS32165 Rev. 4 - 2

13 of 36

www.diodes.com

March 2011

© Diodes Incorporated

A Product Line of

Diodes Incorporated

A basic ZXLD1370 application circuit is shown in Figures 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

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

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

=

=

=

REF

ADJ

S

Rs

LED

V

V

R

mV

225

K

I

I

where K= 0.97

REF

ADJ

S

V

V

R

mV

218

=

S

Rs

LED

R

mV

218

I

I

=

=

Therefore:

LED

I

mV

218

s

R

=

ISM

VIN

REF

ADJ

GI

SGND

Figure 17. Buck configuration

In Boost and Buck-boost mode GI is connected to ADJ through a voltage divider.

current is determined by voltage divider ratio (defined by the resistor

divider) at the GI pin.

⇒

−

=

D

1

I

I

LED

COIL

Standard boost converter equation

D

1

R

I

R

I

V

S

LED

S

COIL

RS

−

×

=

×

=

Sense resistor voltage

Now

=

=

S

REF

ADJ

ADJ

GI

R

mV

225

V

V

V

V

=

S

REF

ADJ

2

GI

1

GI

1

GI

R

mV

225

V

V

)

R

(R

R

+

Rearranging gives

:

REF

ADJ

LED

2

GI

1

GI

1

GI

s

V

V

I

mV

225

)

R

(R

R

R

+

=

ISM

VIN

REF

ADJ

GI

SGND

Figure 18. Boost and Buck-boost connection