ZLED7x20 Datasheet

© 2016 Integrated Device Technology, Inc.

17

April 20, 2016

3.2

Thermal Considerations for Application Design

3.2.1

Temperature Effects of Load, Layout, and Component Selection

Do not exceed the package power dissipation limits by driving high load currents or by operating the chip at high

ambient temperatures. Power dissipation also increases if the efficiency of the circuit is low as could result from

selecting the wrong coil or from excessive parasitic output capacitance on the switch output. See the layout

guidelines in section 3.4.

3.2.2

Temperature Effects of Low Supply Voltage VIN

Until the supply input voltage on the VIN pin has risen above the internally-set startup threshold, the ZLED7x20’s

internal regulator disables the drive to the internal power MOSFET output switch. Above this threshold, the

MOSFET on-resistance is low enough for the chip to start to operate; however, if the supply voltage remains

below the specified minimum (6V), the duty cycle of the output switch will be high and the ZLED7x20 power

dissipation will be at a maximum. Avoid operating the ZLED7x20 under such conditions to reduce the risk of

damage due to exceeding the maximum die temperature. When driving multiple LEDs, their combined forward

voltage drop is typically high enough to prevent the chip from switching when VIN is below 6V, so there is less risk

of thermal damage.

3.3

External Component Selection

Note: Also see section 3.4 for layout guidelines for the following external components.

3.3.1

Sense Resistor Rs

Table 3.1 gives values for Rs under normal operating conditions in the typical application shown in Figure 3.1.

These values assume that the ADJ pin is floating and at the nominal voltage of VREF=1.2V.

Note: Under the conditions given in Table 3.1, in order to maintain the switch current below the maximum value

specified in section 1,

0.082Ω is the minimum value for Rs for the ZLED7020, 0.1Ω for the ZLED7320, 0.13Ω for

the ZLED75

20 and 0.27Ω for the ZLED7720. It is possible to use different values of Rs if the ADJ pin is driven

from an external voltage.

To ensure stable output current, use a 1% accuracy resistor with adequate power tolerance and a good

temperature characteristic for Rs.

Table 3.1

Recommended Values for Sense Resistor Rs (ADJ pin floating at nominal voltage VREF=1.2V)

Nominal Average Output Current (mA)

Value for RS

(Ω)

1200 (maximum for ZLED7020)

0.082

1000 (maximum for ZLED7320)

0.1

750 (maximum for ZLED7520)

0.13

667

0.15

350 (maximum for ZLED7720)

0.27

333

0.3