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参数资料
| 型号： | AN-53 |
| 厂商： | Fairchild Semiconductor Corporation |
| 元件分类： | DC/DC变换器 |
| 英文描述： | Implementing an RC5051 DC-DC Converter on Pentium II Motherboards |
| 中文描述： | 推行RC5051的DC - DC转换器的奔腾II主板 |
| 文件页数： | 13/20页 |
| 文件大小： | 143K |
| 代理商： | AN-53 |
APPLICATION NOTE
AN53
13
RC5051 over-current Characteristics
The RC5051 over-current characteristic includes a hysteresis
function that prevents the DC-DC converter from oscillating
in the event of an over-current. Figure 11 shows the typical
characteristic of the DC-DC converter circuit with a 6m
W
sense resistor. The converter exhibits a normal load regula-
tion characteristic until the voltage across the resistor
exceeds the internal over-current threshold of 120mV. At this
point, the internal comparator trips and signals the controller
to reduce the duty cycle of the high-side MOSFET. This
causes a drastic reduction in output voltage as the load
regulation collapses into the over-current control mode.
The output voltage does not return to its nominal value until
the output current is reduced to a value within the safe range
for the DC-DC converter.
Output Voltage vs. Output Current
RSENSE = 6m
Figure 11. RC5051 Over-current Characteristic
Power Dissipation Consideration During an
Over-current Condition
The RC5051 controller responds to an output over-current by
drastically reducing the duty cycle of the gate drive signal to
the high-side MOSFET. In doing this, the high-side MOS-
FET is protected from stress and from eventual failure. Fig-
ure 12A shows the gate drive signal of a typical RC5051
operating in continuous mode with a load current of 10A.
The duty cycle is set by the ratio of the input voltage to the
output voltage. If the input voltage is 5V, and the output volt-
age is 2.8V, the ratio of Vout/ Vin is 56% (64% measured).
Figure 12B shows the result of a RC5051 going into its over-
current mode with a duty cycle of approximately 47%. Cal-
culating the power in each MOSFET at each condition on the
graph (Figure 11) shows how the protection works. The
power dissipated in the high-side MOSFET at normal opera-
tion for a load current of 14.2A, is given by:
ignoring switching losses.
The power dissipated in the MOSFET at an over-current
condition of 20A, is given by:
again ignoring switching losses.
These calculations show that the high-side MOSFET is not
being over-stressed during an over-current condition.
Figure 12A. HIDRV Output Waveform for Normal
Operation Condition with V
out
= 2.8V@10A
Figure 12B. HIDRV Output Waveform for
Over-current Condition
Power dissipation in the low-side MOSFET during an over-
current condition must also be considered. The low-side
MOSFET dissipates power while the high-side MOSFET is
off. The power dissipated in the low-side MOSFET during
normal operation, is given by:
During an over-current , the duty cycle reduces to around
47%. The power dissipated in the low-side MOSFET during
short circuit condition, is given by:
Thus, for the low-side MOSFET, the thermal dissipation dur-
ing over-current is greatly magnified. This requires that the
thermal dissipation of the low-side MOSFET be properly
managed by an appropriate heat sink. To protect the low-side
MOSFET from being destroyed in the event of an over-cur-
1.0
0.5
0
1.5
2.0
2.5
3.0
3.5
0
5
10
15
20
25
6
Output Current (A)
O
P
D
I
2
R
DS ON
′
DC
14.2
(
)
2
.01
.64
′
1.29W
=
′
=
′
=
P
D
20
(
)
2
.01
′
.47
′
1.88W
=
=
I
2
RRDS ON
′
1
DC
–
(
)
14.2
(
)
2
.01
.36
′
0.73
=
′
=
′
=
P
D
20
(
)
2
.01
′
.53
′
2.1W
=
=
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