參數(shù)資料
型號: M7085L-SM1-R
廠商: UNISONIC TECHNOLOGIES CO LTD
元件分類: 穩(wěn)壓器
英文描述: PFM STEP-DOWN DC-DC CONTROLLER
中文描述: SWITCHING CONTROLLER, PDSO8
封裝: LEAD FREE, MSOP-8
文件頁數(shù): 7/8頁
文件大?。?/td> 139K
代理商: M7085L-SM1-R
M7085
CMOS IC
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
7 of 8
QW-R502-092,A
APPLICAT ION INFORMAT ION(Cont.)
Setting over current protection threshold by external sense resistor
The V
DS
of a P-Channel will tend to vary significantly over temperature. This will result an equivalent variation in
current limit. To improve current limit accuracy an external sense resistor can be connected from V
IN
to the source of
the P-Channel, as shown in Figure 2.
I
SENSE
COMP
5.5μA
ADJ
5
C
ADJ
R
ADJ
I
SENSE
V
IN
FIGURE 2. Current Sensing by External Resistor
1
Setting start up time
The current limit circuit is active during start-up. During start-up the P-Channel will stay on until either the current
limit or the feedback comparator is tripped If the current limit comparator is tripped first then the fold back
characteristic should be taken into account. Start-up into full load may require a higher current limit set point or the
load must be applied after start-up.
One problem with selecting a higher current limit is inrush current during start-up. Increasing the capacitance(C
ADJ
)
in parallel with R
ADJ
results in soft-start. C
ADJ
and R
ADJ
create an RC time constant forcing current limit to activate at
a lower current. The output voltage will ramp more slowly when using the soft-start functionality. There are example
start-up plots for C
ADJ
equal to 1nF and 10nF in the Typical Performance Characteristics. Lower values for C
ADJ
will
have little to no effect on soft-start.
Inductor selection (L1)
UTC
M7085
operates over a wide frequency range and can use a wide range of inductance values. The
inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The second is
the ESR.
Output capacitor selection (C
OUT
)
The output capacitor, C
OUT
, is required to keep the output voltage ripple small and to ensure regulation loop
stability. C
OUT
must have low impedance at the switching frequency. For most applications, a 200
Μ
f capacitor is
sufficient.
Input capacitor selection (C
IN
)
The input capacitor, C
IN
, reduces the current peaks drawn and reduces switching noise in the IC. a bypass
capacitor is required between the input source and ground. It must be located near the source pin of the external
P-Channel. The input capacitor prevents large voltage transients at the input and provides the instantaneous current
when the P-Channel turns on. The capacitance value should be selected such that the ripple voltage created by the
charge and discharge of the capacitance is less than 10% of the total ripple across the capacitor.
Catch diode selection (D1)
The important parameters for the catch diode are the peak current, the peak reverse voltage, and the average
power dissipation. For high temperature applications, diode leakage current may become significant and require a
higher reverse voltage rating to achieve acceptable performance.
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