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| 型号： | MC34652EFR2 |
| 厂商： | Freescale Semiconductor |
| 文件页数： | 19/26页 |
| 文件大小： | 1802K |
| 描述： | IC CTRLR HOT SWAP NEG V 16-SOIC |
| 标准包装： | 2,500 |
| 类型： | 热交换开关 |
| 应用： | 通用 |
| 内部开关： | 是 |
| 电流限制： | 2A |
| 电源电压： | 15 V ~ 80 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 16-SOIC N |
| 包装： | 带卷 (TR) |
Analog Integrated Circuit Device Data
Freescale Semiconductor
19
34652
TYPICAL APPLICATIONS
rchived by Freescale Semiconductor, Inc., 2008
timer initiates. The UV and OV detection circuits are
equipped with a 1.0 ms filter to filter out momentary input
supply dips. Filter capacitors between the UV and VIN pins
and between the OV and VIN pins could also be added to
adjust the UV/OV filter time and prevent more transients from
affecting the devices operation, especially if the input supply
has a lot of noise.
Guidelines for Choosing Resistor Divider Values
The total current flowing in the resistors is equal to the
supply voltage divided by the total series resistance. The
supply voltage can reach up to 80 V and the device will still
be in normal operation, the resistors connected and drawing
current. So the resistor values should be chosen high enough
to allow for a reasonable current to pass through them and
not dissipate a lot of power or cause input noise that would
trip the UV / OV detection circuit.
Another consideration is whether or not the values of the
resistors are readily available. The tolerance of the resistors
should be 1% or better to get an accurate reading.
Note Accuracy requirements are application dependent.
To demonstrate the importance of the accuracy of the
resistors, lets look at a system with an operating range of
40 V for UV falling to 75 V for OV rising as an example. This
operating range will be scaled down for the devices internal
circuitry to operate the UV/OV detection circuits. The scale
factor is 31.6 for UV and 57.1 for the OV. Taking overvoltage
as an example, this means that every 5.0 mV change on the
OV pin represents a 0.29 V change for the OV trip point on
the supply. Which says that an error of 5.0 mV due to the
resistors not being accurate will result in an error of 0.29 V for
the trip point, and depending on how close we are operating
to the OV rising threshold the device might detect an OV
condition and turn off the Power MOSFET prematurely. The
same argument applies to the UV pin.
Example of Calculations for Resistor Values
The following equations are examples of calculating
resistor values using the same operating range as in the
previous paragraph:
R
3
= 1.3
*
R
1
*
V
UV
(RISING)
/ (V
OV (RISING)
(V
UV (RISING)
- 1.3))
R
2
= R
3
(V
OV (RISING)
/ V
UV (RISING)
- 1)
Where V
OV (RISING)
= 75 V and V
UV (RISING)
= 41 V
Note Some iteration may be required to get the right
values and also standard resistor values. The recommended
maximum value of the series resistance between the UV/OV
pins and VPWR
pin is 500 k?
Here we have two equations and three unknowns. If we
select a value for R
1
of 487 k? then from the first equation:
R
3
= 8.72 k?/DIV>
and the closest 1% standard resistor value is 8.66 k?
Now, from the second equations we can solve for R
2
:
R
2
= 7.18 k?/DIV>
and the closest 1% standard resistor value is 7.15 k?
If the three-resistor divider, which is the recommended
approach, could not produce acceptable resistor values, the
user can consider two separate resistor dividers, one divider
for each pin from VPWR to VIN. An advantage of the two-
resistor dividers approach is that the user can set the trip
points of the UV and OV thresholds independently.
TIMER
The TIMER pin on the 34652 gives the user control over
the time base used to generate the timing sequences at start-
up. The same timer controls the retry delay when the device
experiences any fault. The TIMER pin can be left
unconnected for a default timer value of 200 ms or the user
can connect an external resistor (R
TIMER
) between the
TIMER and VIN pins, as shown in Figure 19
, page 18
, to set
the timer value externally.
After the device passes the UVLO threshold and
continuously after that, the 34652 checks the TIMER pin for
any external components to determine the value of the timer.
During start-up and if any fault occurred, this timer value is
used when initiating a start-up sequence.
Choosing the External Resistor R
TIMER
Value
The user can change the value of the Start-Up Delay
Timer (t
TIMER
) by adding a resistor (R
TIMER
) between the
TIMER and VIN pins, as shown in Figure 19
, page 18
. The
timer value ranges between 100 ms and 1000 ms, with a
default value of 200 ms. Table 6
lists examples of R
TIMER
for
different values of the t
TIMER
and Figure 20
, page 20
, shows
a plot of R
TIMER
versus t
TIMER
. It is recommended that the
closest 1% standard resistor value to the actual value be
chosen.
Note Accuracy requirements are application dependent.
To calculate the value of the R
TIMER
resistor we use the
following equations:
t
TIMER
(ms) = 20 (ms) + 2.0
*
[R
TIMER
(k? + 1.0 k
R
TIMER
(k?
= [t
TIMER
(ms)
- 20 (ms) ] / 2.0 - 1.0 k?/DIV>
Table 6. R
TIMER
Values for Some Desired t
TIMER
Values
t
TIMER
(ms) R
TIMER
(k?/SPAN>) t
TIMER
(ms) R
TIMER
(k?/SPAN>)
100
39
600
289
150
64
650
314
200
89
700
339
250
114
750
364
300
139
800
389
350
164
850
414
400
189
900
439
450
214
950
464
500
239
1000
489
500
264
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