AN2928 datasheet(4/21 Pages) STMICROELECTRONICS

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Modified buck converter in constant current mode

AN2928

4/21

Modified buck converter in constant current mode

As stated in the introduction, the aim of this application note is to describe a modified buck

converter working in FOT and CCM. The basic principle of the design using the L6562A

controller is shown in Figure 1 and Figure 2. Figure 1 represents the stage when the power

MOSFET Q1 is turned on. As shown by the red arrow, the current flows from the DC voltage

input (VIN) through the load (LEDs), the inductor (L), the power MOSFET Q1 and the

sensing resistor. Capacitor C4 is charged via diode D2 and resistor R5, since the transistor

Q1 is open and its gate voltage is around 10 V. During the tON time, the load current

increases and stops as soon as the voltage on the current sense resistor reaches the

internal threshold on the CS pin of the L6562A. The current sense of the L6562A is clamped

at 1.08 V (typ). Figure 2 shows the tOFF time, when the power MOSFET is switched off. The

inductor keeps the current flowing in the same direction and the circuit is closed through

diode D1. The load current is decreasing and the minimum current is set by the fixed off-time

network (tOFF time is always constant), because capacitor C4 is discharged to the resistor

R4. The voltage on capacitor C4 is connected to the ZCD (zero current detector) pin of the

L6562A. As soon as the capacitor is discharged and its voltage falls below 0.7 V (the ZCD

threshold), the L6562A switches the power MOSFET again and the load current is

increased. This process repeats cycle-by-cycle, as shown in the timing diagrams in Figure 1

and Figure 2.

Figure 1.

Modified buck converter - tON time

I_MAX

I_AVR

I_MIN

GND

COMP

INV

VIN

LEDs

load

Fixed off-time

network

I LED

FOT

tOFF

tON

tON tOFF

FOT- fixed off-time

tON

MULT

L6562A

VCC

ZCD

AM00366

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