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參數(shù)資料
| 型號： | FAN5193 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | Two Phase Interleaved Synchronous Buck Converter |
| 中文描述： | 兩相交錯同步降壓轉(zhuǎn)換器 |
| 文件頁數(shù)： | 15/21頁 |
| 文件大小： | 524K |
| 代理商： | FAN5193 |
PRODUCT SPECIFICATION
FAN5193
REV. 1.0.1 2/4/02
15
Current Sharing
The two independent current sensors of the FAN5193 operate
with their independent current control loops to guarantee that
the two phases each deliver half of the total output current.
The main source of mismatch between the two phases occurs
if there is a mismatch between the R
DS,on
of the low-side
MOSFETs. Increased amounts of droop can also assist in
current balancing. For this reason, current balance is best at
maximum load current; at very light loads, there may be
significant mismatch, but there is no circulating current
because of the backfeed protection circuit.
Short Circuit Current Characteristics
The FAN5193 short circuit current characteristic includes a
function that protects the DC-DC converter from damage in
the event of a short circuit. The short circuit limit is set with
the R
S
resistor, as given by the formula
with I
SC
the desired current limit, RT the oscillator resistor
and R
DS,on
one phase’s low-side MOSFET’s on resistance.
Remember to make the R
S
large enough to include the effects
of initial tolerance and temperature variation on the
MOSFETs’ R
DS,on
. It is recommended to set I
SC
substan-
tially above maximum operating current, to avoid nuisance
trips.
Important Note!
The oscillator frequency must be selected
before selecting the current limit resistor, because the value
of RT is used in the calculation of R
S
.
When an overcurrent is detected, the high-side MOSFETs
are turned off, and the low-side MOSFETs are turned on,
and they remain in this state until the measured current
through the low-side MOSFET has returned to zero amps.
After reaching zero, the FAN5193 re-soft-starts, ensuring
that it can also safely turn on into a short.
A limitation on the current sense circuit is that I
SC
R
DS,on
must be less that 375mV. To ensure correct operation, use
I
SC
R
DS,on
≤
300mV; between 300mV and 375mV, there
will be some non-linearity in the short-circuit current not
accounted for in the equation.
As an example, consider the typical characteristic of the
DC-DC converter circuit with two FDP6670AL low-side
MOSFETs (R
DS
= 6.5m
maximum at 25
°
C 1.2 at 75
°
C
= 7.8m
each, or 3.9m
total) in each phase, RT = 42.1K
(600KHz oscillator) and a 50K
R
S
.
The converter exhibits a normal load regulation characteris-
tic until the voltage across the MOSFETs exceeds the inter-
nal short circuit threshold of 50K
/(3.9m
41.2K
6.66)
= 47A. [Note that this current limit level can be as high as
50K
/(3.5m
41.2K
6.66) = 52A, if the MOSFETs
have typical R
DS,on
rather than maximum, and are at 25
°
C.]
At this point, the internal comparator trips and signals the
controller to leave on the low-side MOSFETs and keep off
the high-side MOSFETs. The inductor current decreases,
and power is not applied again until the inductor current
reaches 0A and the converter attempts to re-softstart.
Precision Current Sensing
The tolerances associated with the use of MOSFET current
sensing can be circumvented by the use of a current sense
resistor, as provided for by the FAN5094.
Light Load Ef
fi
ciency
At light load, the FAN5193 uses a number of techniques to
improve efficiency. Because a synchronous buck converter is
two quadrant, able to both source and sink current, during
light load the inductor current will flow away from the out-
put and towards the input during a portion of the switching
cycle. This reverse current flow is detected by the FAN5193
as a positive voltage appearing on the low-side MOSFET
during its on-time. When reverse current flow is detected, the
low-side MOSFET is turned off for the rest of the cycle, and
the current instead flows through the body diode of the
high-side MOSFET, returning the power to the source. This
technique substantially enhances light load efficiency.
E*-mode
In addition, further enhancement in efficiency can be obtained
by putting the FAN5193 into E*-mode. When the Droop pin
is pulled to the 5V BYPASS voltage, the “A” phase of the
FAN5193 is completely turned off, reducing in half the
amount of gate charge power being consumed. E*-mode can
be implemented with the circuit shown in Figure 5:
Figure 5. Implementing E*-mode Control
Note that the charge pump for the HIDRVs should be based
on the “B” phase of the FAN5193, since the “A” phase is off
in E*-mode.
Internal Voltage Reference
The reference included in the FAN5193 is a precision band-
gap voltage reference. Its internal resistors are precisely
trimmed to provide a near zero temperature coefficient (TC).
Based on the reference is the output from an integrated 5-bit
DAC. The DAC monitors the 5 voltage identification pins,
VID0-4, and scales the reference voltage from 1.100V to
1.850V in 25mV steps.
R
S
(
)
I
SC
R
DS on
RT
6.66
=
BYPASS
10K
1K
10K
2N2222
2N2907
R
DROOP
FAN5093
pin 21
HI = E*-
mode on
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