Fairchild RC5050 Schottky Diode Selection Table, Output Filter Capacitors, Manufacturer

AN50 APPLICATION NOTE

Figure 13A. VCCQP Output Waveform for Normal

Operation Condition with Vout = 3.3V@10A

Figure 13B. VCCQP Output Waveform for

Output Shorted to Ground

Power dissipation on the Schottky diode during a short cir- cuit condition must also be considered. During normal oper- ation, the Schottky diode dissipates power while the power MOSFET is off. The power dissipated in the diode during normal operation, is given by:

PD, Diode = IF × VF × (1 – DutyCycle) =

14.5 × 0.5V × (1 – 0.62) = 2.75W

During a short circuit, the duty cycle dramatically reduces to around 20%. The forward current in the short circuit condi- tion decays exponentially through the inductor. The power dissipated in the diode during short circuit condition, is approximately given by:

–	-----------1	–	1.5------------	µs-
IF, ending = Isc × e	L ⁄ R = 20A × e		1.3	µs ≈ 7.9A

IF, ave ≈ (20A + 7.9A) ⁄ 2 ≈ 14A

PD, Diode = IF, ave × VF × (1 – DutyCycle) =

14 × 0.45 × 0.8 ≈ 5W

Thus, for the Schottky diode, the thermal dissipation during a short circuit is greatly magnified. This requires that the thermal dissipation of the diode be properly managed by an appropriate heat sink. To protect the Schottky from being destroyed in the event of a short circuit, you should limit the junction temperature to less than 130°C. You can find the required thermal resistance using the equation for maximum junction temperature:

PD	TJ(max) – TA
	= -------------------------------
	RΘJA

Assuming that the ambient temperature is 50°C,

RΘJA	TJ(max) – TA	=	130 – 50	= 16°C ⁄ W
	= -------------------------------		-------------------5-
	PD

Thus, you need to provide a heat sink that gives the Schottky diode a thermal resistance of 16°C/W or lower to protect the device during an indefinite short.

In summary, with proper heat sink, the Schottky diode is not over-stressed during a short circuit condition.

Schottky Diode Selection

The application circuit diagram of Figure 3 shows a Schottky diode, DS1. In non-synchronous mode, DS1 is used as a fly- back diode to provide a constant current path for the inductor when M1 is turned off. Table 10 shows the characteristics of several Schottky diodes. Note that MBR2015CTL has a very low forward voltage drop. This diode is ideal for applications where the output voltage is required to be less than 2.8V.

Table 10. Schottky Diode Selection Table

Manufacturer		Forward Voltage
Model #	Conditions	VF
Philips	IF = 20A; Tj = 25°C	< 0.84v
PBYR1035	IF = 20A; Tj = 125°C	< 0.72v
Motorola	IF = 20A; Tj = 25°C	< 0.84v
MBR2035CT	IF = 20A; Tj = 125°C	< 0.72v
Motorola	IF = 15A; Tj = 25°C	< 0.84v
MBR1545CT	IF = 15A; Tj = 125°C	< 0.72v
Motorola	IF = 20A; Tj = 25°C	< 0.58v
MBR2015CTL	IF = 20A; Tj = 150°C	< 0.48v

Output Filter Capacitors

Output ripple performance and transient response are func- tions of the filter capacitors. Since the 5V supply of a PC motherboard may be located several inches away from the DC-DC converter, the input capacitance may play an impor- tant role in the load transient response of the RC5050 and RC5051. The higher input capacitance, the more charge stor- age is available for improving current transfer through the

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