Fairchild RC5042, RC5040 specifications Schottky Diode Selection

AN42		APPLICATION NOTE
	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 and requires that the
	thermal dissipation of the diode be properly managed by the
	appropriate choice of a heat sink. In order to protect the
	Schottky from being destroyed in the event of a short, we
	should limit the junction temperature to less than 130°C.
	Using the equation for maximum junction temperature,
	we can arrive at the thermal resistance required below:
	TJ(max) – TA
	PD = -------------------------------
Figure 14A. VCCQP Output Waveform for Normal	RΘJA
	Assuming that the ambient temperature is 50°C, we get:
Operation Condition with Vout = 3.3V@10A
	TJ(max) – TA	130 – 50
	RΘJA = -------------------------------	= --------------------= 16°C ⁄ W
	PD	5

Thus we need to provide for a heat sink that will give the Schottky diode a thermal resistance of at least 16°C/W or lower in order to protect the device during an indefinite short.

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

Figure 14B. VCCQP Output Waveform for

Output Shorted to Ground

The Schottky diode has a power dissipation consideration during the short circuit condition. During normal operation, the diode dissipates power when the power MOSFET is off. The power dissipation is given by:

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

14.5 ⋅ 0.5V ⋅ (1 – 0.62) = 2.75W

In short circuit mode, the duty cycle is dramatically reduced to approximately 20%. The forward current during a short circuit condition decays exponentially through the inductor. The power dissipated on the diode during the short circuit condition, is approximated by:

1	1.5us
IF, ending = Isc ⋅ e– L-----------⁄R = 20A ⋅ e–	1.3us-------------≈ 7.9A
IF, ave ≈ (20A + 7.9A) ⁄ 2 ≈ 14A

Schottky Diode Selection

The application circuits of Figures 3, 4, and 5 show two Schottky diodes, DS1 and DS2. In synchronous mode, DS1 is used in parallel with M3 to prevent the lossy diode in the FET from turning on. In non-synchronous mode, DS1 is used as a flyback diode to provide a constant current path for the inductor when M1 is turned off.

The Schottky diode DS2 serves a dual purpose. As config- ured in Figures 3, 4, and 5, DS2 allows the VCCQP pin on the RC5040 to be bootstrapped up to 9V using capacitor C12. When the lower MOSFET M3 is turned on, one side of capacitor C12 is connected to ground while the other side of the capacitor is being charged up to voltage VIN – VD through DS2. The voltage that is then applied to the gate of the MOSFET is VCCQP – VSAT, or typically around 9V. DS2 also provides correct sequencing of the various supply voltages by assuring that VCCQP is not enabled before the other supplies.

A vital selection criteria for DS1 and DS2 is that they exhibit a very low forward voltage drop, as this parameter can directly affect the regulator efficiency. Table 10 lists several suitable Schottky diodes. Note that the MBR2015CTL has a very low forward voltage drop. This diode is ideal for appli- cations where output voltages less than 2.8V are required.

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