CAT4201

www.onsemi.com

8

Simplified Block Diagram

Figure 22. CAT4201 Simplified Block Diagram

1.2 V

Reference

+

–

ON−Time Control

OFF−Time Control

1.2 V

1

W

7 V

30 k

W

20

mA

EN

SW

GND

RSET

CTRL

VBAT

12 V/24 V

PWM

Controller

EN

Basic Operation

The CAT4201 is a high efficiency step−down regulator

designed to drive series connected high−power LEDs. LED

strings with total forward voltages of up to 32 V can be

driven with bias currents of up to 350 mA.

During the first switching phase, an integrated high

voltage power MOSFET allows the inductor current to

charge linearly until the peak maximum level is reached, at

which point the MOSFET is switched off and the second

phase commences, allowing the inductor current to then

flow through the Schottky diode circuit and discharge

linearly back to zero current.

The switching architecture ensures the device will always

operate at the cross−over point between Continuous

Conduction Mode (CCM) and Discontinuous Conduction

Mode (DCM). This operating mode results in an average

LED current which is equal to half of the peak switching

current.

LED Pin Current

The LED current is set by the external RSET resistor

connected to the regulated output of the RSET pin. An

overall current gain ratio of approximately 2.5 A/mA exists

between the average LED current and the RSET current,

hence the following equation can be used to calculate the

LED current.

LED Current (A)

^ 2.5

V

R

Table 7 lists the various LED currents and the associated

RSET resistors.

Table 7. RSET RESISTOR SELECTION

LED Current (A)

RSET (k

W)

0.10

33

0.15

21

0.20

15

0.25

12

0.30

10

0.35

8.25