CAT4201_15 Datasheet, PDF (8/12 Pages) ON Semiconductor – High Efficiency Step Down LED Driver

Simplified Block Diagram
12 V/24 V
CAT4201
CTRL
20 mA
30 kW
VBAT
7V
OFF−Time Control
SW
EN
PWM
Controller
1.2 V
Reference
+
EN
–
1.2 V
ON−Time Control
R₂
1W
RSET
GND
Figure 22. CAT4201 Simplified Block Diagram
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_RSET (V)
R_SET (kW)
Table 7 lists the various LED currents and the associated
RSET resistors.
Table 7. RSET RESISTOR SELECTION
LED Current (A)
RSET (kW)
0.10
33
0.15
21
0.20
15
0.25
12
0.30
10
0.35
8.25
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