Design considerations and component selection, Short circuit protection, Oscillator – Fairchild SEMICONDUCTOR RC5051 User Manual
Page 5
APPLICATION NOTE
AN50
5
Short Circuit Protection
A current sense methodology is implemented to disable the
output drive signal to the MOSFET(s) when an over-current
condition is detected. The voltage drop created by the output
current flowing across a sense resistor is presented to an
internal comparator. When the voltage developed across the
sense resistor exceeds the comparator threshold voltage, the
chip reduces the output drive signal to the MOSFET(s).
The DC-DC converter returns to normal operation after the
fault has been removed, for either an over-voltage or a short
circuit condition.
Oscillator
The RC5050 and RC5051 oscillator section uses a fixed cur-
rent capacitor charging configuration. An external capacitor
(C
EXT
) is used to preset the oscillator frequency between
200KHz and 1MHz. This scheme allows maximum flexibil-
ity in setting the switching frequency and in choosing exter-
nal components.
In general, a lower operating frequency decreases the peak
ripple current flowing in the output inductor, thus allowing
the use of a smaller inductor value. Unfortunately, operation
at lower frequencies increases the amount of energy storage
that must be provided by the bulk output capacitors during
load transients due to slower loop response of the controller.
In addition, the efficiency losses due to switching of the
MOSFETs increase as the operating frequency is increased.
Thus, efficiency is optimized at lower operating frequencies.
An operating frequency of 300 kHz was chosen to optimize
efficiency while maintaining excellent regulation and tran-
sient performance under all operating conditions.
Design Considerations and
Component Selection
Figure 3 shows a typical non-synchronous application using
the RC5050. Figure 4 illustrates the synchronous applica-
tion using the RC5051.
Figure 3. Non-Synchronous DC-DC Converter Application Schematic Using the RC5050
VO
GND
VID3
VID2
VID1
VID0
8
7
6
5
4
3
2
1
9
10
11
12
13
14
15
16
17
18
19
20
+5V
PWRGD
RC5050
VCC
ENABLE
C10
0.1
µ
F
C11
0.1
µ
F
10K
C
EXT
C7
0.1
µ
F
C6
4.7
µ
F
DS1
MBR2015CTL
C1
1000
µ
F
C2
C3
1000
µ
F 1000
µ
F
C5
0.1
µ
F
D1
1N4691
C12
1
µ
F
L1
1.3µ
H
M1
IRF7413
M2
IRF7413
C8
C9
0.1
µ
F
0.1
µ
F
R
SENSE
6m
Ω
C13
1500
µ
F
1500
µ
F
L2
2.5
µ
H
VREF
C14
1500
µ
F
C4
0.1
µ
F
1500
µ
F
C15
C16
+12V
VID4
47
R5
R6
100pF