Fairchild RC5050, RC5051 Schottky Diode Selection, Table 10. Schottky Diode Selection Table, Output Filter Capacitors

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:

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:

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:

Assuming that the ambient temperature is 50°C,

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 TableOutput 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

PD Diode,IFVF

×1 DutyCycle–( )× = =

14.5 0.5V×1 0.62–( ) 2.75 W=×

IF ending,Is c e

L R⁄

-----------–

×20A e

1.5µs

1.3µs

--------------–

×7.9A≈==

IF ave,20A 7.9 A+( ) 2⁄14A≈ ≈

Manufacturer

Model # Conditions Forward Voltage

Philips

PBYR1035 IF = 20A; Tj = 25°C

IF = 20A; Tj = 125°C< 0.84v

< 0.72v

Motorola

MBR2035CT IF = 20A; Tj = 25°C

IF = 20A; Tj = 125°C< 0.84v

< 0.72v

Motorola

MBR1545CT IF = 15A; Tj = 25°C

IF = 15A; Tj = 125°C< 0.84v

< 0.72v

Motorola

MBR2015CTL IF = 20A; Tj = 25°C

IF = 20A; Tj = 150°C< 0.58v

< 0.48v

PD Diode,IF a ve,VF

×1 DutyCycle–( )× = =

14 0.45×0.8×5 W≈

TJ max( ) TA

–

RΘJA

-------------------------------=

RΘJA

TJ max( ) TA

–

------------------------------- 130 50–

--------------------- 16°C W⁄= = =