ZLED7001 Datasheet

© 2016 Integrated Device Technology, Inc.

8

April 20, 2016













+

∗

=

pF

10

C

1

ns

510

T

OFF

TIME

_

OFF

LED

CSTH

CS

I

15

.

1

V

R

∗

=

2.4.

Current Sensing

Assuming a 30% current ripple in the inductor, the sense resistor RCS can be calculated as shown in equation (3):

(3)

Where

VCSTH = 240mV (typical)

The current sense input of the ZLED7001 is connected to the non-inverting inputs of two comparators. The

inverting terminal of one comparator is tied to an internal 240mV reference and the inverting terminal of the other

comparator is connected to the LD pin. The outputs of both comparators are fed into an OR gate, and the output

of the OR gate is connected to the reset pin of the flip-flop. Thus, the comparator that has the lower voltage at the

inverting input determines when the GATE output is turned off.

The comparator outputs also include a typical 480ns blanking time that prevents spurious turn-offs of the external

MOSFET due to the turn-on spike normally present as a result of transistor gate-source capacitance. In rare

cases, this internal blanking time might not be enough to filter out the turn-on spike. If so, an external RC filter

must be added between the external sense resistor (RCS) and the CS pin.

Note that the comparators are relatively fast: 80ns typical response time. Invalid triggering by these comparators

could result if the layout fails to minimize external inductances.

2.5.

Timing Circuit

The timing circuit in the ZLED7001 is controlled by a single capacitor connected from TOFF to ground.

TOFF, the time of the cycle period, is given by equation (4):

(4)

2.6.

PWM Dimming Application Circuit

For PWM dimming applications, the ZLED7001’s PWMD pin is driven with a low-frequency square-wave control

signal. The GATE pin’s driver is enabled when the control signal is high and disabled when the control signal is

low. The LED current’s rise and fall rate is controlled only by the inductance value, the supply voltage, and LED

forward voltage.

If the PWMD pin is allowed to float, the PWM dimming function is disabled.

2.7.

Linear Dimming Application Circuit

For linear dimming applications, an external voltage ranging from 50mV (typical) to 240mV (typical) is applied to

the LD (linear dimming) pin to control the LED current during operation. Linear dimming can be used to adjust the

LED current level to reduce the LED’s brightness. In this case, connect a resistor between the VREF pin and the

LD pin and connect a negative-temperature-coefficient (NTC) thermistor between the LD pin and ground. The

ZLED7001 can also provide temperature compensation, (see the application circuit on page 1 and section 2.8).