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參數(shù)資料
| 型號： | UC1914 |
| 廠商： | Texas Instruments, Inc. |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | 5V to 35V Hot Swap Power Manager |
| 中文描述： | 5V至35V的熱插拔電源管理器 |
| 文件頁數(shù)： | 11/14頁 |
| 文件大?。?/td> | 652K |
| 代理商： | UC1914 |
11
UC1914
UC2914
Resistive Load:
CT
min
=
(
)
(
)
R
V
I
R
T
R
V
R
2
PL
CC
MAX
LOAD
START
PL
CC
PL
+
+
10
2
4
R
C
LOAD
LOAD
Output Current Softstart
The external MOSFET output current can be increased
at a user-defined rate to ensure that the output voltage
comes up in a controlled fashion by adding capacitor
C
SS
, as shown in Fig. 5. The chip does place one con-
straint on the soft start time and that is that the charge
pump time constant has to be much less than the
softstart time constant to ensure proper soft start opera-
tion. The time constant determining the startup time of
the charge pump is given by:
τ
CP
OUT
R
PUMP
C
=
R
OUT
is the output impedance of the charge pump given
by:
1
R
f
CP
OUT
PUMP
=
where f
PUMP
is the charge pump frequency (125kHz) and
CP = CP1 = CP2 are the charge pump flying capacitors.
For typical values of CP1, CP2 and C
PUMP
(0.01
μ
F) and
a switching frequency of 125kHz, the output impedance
is 800
and the charge pump time constant is 8
μ
s. The
charge pump should be close to being fully charged in 3
time constants or 24
μ
s. By placing a capacitor from VCC
to IMAX, the voltage at IMAX, which sets the maximum
output current of the FET, will exponentially decay from
VCC to the desired value set by R1 and R2. The output
current of the MOSFET will be controlled via soft start as
long as the soft start time constant (
τ
SS
) is much greater
than the charge pump time constant
τ
CP
, given by
(
)
SS
Minimizing Total Dropout Under Low Voltage
Operation
τ
SS
R
R
C
=
1
2
In a typical application, the UC3914 will be used to con-
trol the output current of an external NMOS during hot
swapping situations. Once the load has been fully
charged, the desired output voltage on the load, V
OUT
,
will be required to be as close to VCC as possible to
minimize total dropout. For a resistive load, R
LOAD,
the
output voltage is given by:
R
R
R
LOAD
SENSE
+
V
R
VCC
OUT
LOAD
DS on
=
+
(
)
R
SENSE
was picked to set the fault current, I
FAULT
.
R
DS(on)
, the on-resistance of the NMOS, should be made
as small as possible to ensure V
OUT
is as close to VCC
as possible. For a given NMOS, the manufacturer will
specify the R
DS(on)
for a certain V
GS
(maybe 7V to 10V).
The source potential of the NMOS is V
OUT.
In order to
ensure sufficient VGS, this requires the gate of the
NMOS, which is the output of the linear amplifier, to be
many volts higher than VCC. The UC3914 provides the
capability to generate this voltage through the addition of
3 capacitors, CP1, CP2 and C
PUMP
as shown in Fig. 6.
These capacitors should be used in conjunction with the
complementary output drivers and internal diodes in-
cluded on-chip to create a charge pump or voltage
tripler. The circuit boosts VCC by utilizing capacitors
CP1, CP2 and C
PUMP
in such a way that the voltage at
V
PUMP
approximately equals (3 V
CC
) – (5 V
DIODE
), al-
most tripling the input supply voltage to the chip.
APPLICATION INFORMATION (cont.)
6
11
12
9
8
7
C
PUMP
L
O
A
D
C
LOAD
V
OUT
M1
2
OUT
VOUTS
VPUMP
VCC
R2
18
16
C
SS
R1
C1
IMAX
REF
OSC
PMP
5
OSCB
PMPB
CP2
CP1
Figure 5. MOSFET soft start diagram.
UDG-98160
Figure 6. Charge pump block diagram.
UDG-97058
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