IRPLLED1A International Rectifier, IRPLLED1A Datasheet - Page 11

• Manufacturer Part Number: IRPLLED1A
• Description: Power Management IC Development Tools Hi-Vlt DC-DC Buck Cnvr HBLED Cur Cntrl
• Manufacturer: International Rectifier
• Type: DC/DC Converters, Regulators & Controllersr

Specifications of IRPLLED1A

• Product: Power Management Development Tools
• Tool Is For Evaluation Of: IRS25401PBF
• Output Voltage: 16 V to 24 V
• Output Current: 350 mA to 1.5 A
• Factory Pack Quantity: 1

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
IRS25401/11 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 IRPLLED1A 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 IRPLLED1A 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 IRS25401/11 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.
RD-0608
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