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RD-0608

Fig. 15 Iout = 350mA, Vin = 100V, Vout = 16.85V, L = 470uH, COUT = 33uF

Since the system is operating in a free running hysteretic mode, the frequency will adjust to whatever point

maintains the regulation of the load current. Since there are fixed delays and a fixed dead time within the

IRS2540/1/01/11 controller, the current monitored at the IFB input always overshoots the threshold before the

system switches from the energy store phase to the energy release phase. The current must also undershoot the

threshold before the system switches back to the energy storage phase. These inherent delays in the system mean

that the inductor L1 and capacitor COUT (if used) must be selected to maintain a low enough frequency to minimize

this overshoot and undershoot. Output ripple is determined by overshoot and undershoot, which become greater as

frequency increases.

The inductor for the IRPLLED1 evaluation board was selected to supply the maximum output current of 1.5A. Due

to constraints of size a value of 470uH was chosen, which provides good performance. The IRPLLED1 may be used

with or without the output capacitor COUT connected. If COUT is not connected the system runs at higher

frequency and consequently with reduced efficiency due to switching losses.

Because of these considerations an inductor of 470uH and an output capacitance of 33uF were chosen to

accommodate the 1.5A load current. The current ripple associated with 470uH is relatively small so the board can

be operated with or without output capacitance at the lower current ratings.

5. MOSFET vs Diode for the low side switch

The IRS2540/1/01/11 has been designed so that it can drive a low side MOSFET and a high side MOSFET.

Alternatively the low side FET can be replaced by a freewheeling diode as shown in Fig. 16. This may yield a lower

cost system, but there are some efficiency tradeoffs to be considered, particularly for higher load currents. The

system efficiency is directly influenced by several system parameters including operating frequency, load current,

and input voltage.

In this evaluation board a fast recovery power diode is used for the lower switch.

A major parameter to consider is the reverse recovery time of the diode in comparison to the body diode of the FET

it replaces. The diode intrinsically has a much shorter reverse recovery time since the device is specifically designed

for this, where as the body diode is a parasitic element that originates from basic processing technology and

typically has inferior characteristics, in terms of forward drop, reverse recovery, and power handling capabilities.