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參數(shù)資料
| 型號： | LT1683 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Ultralow Noise Push-Pull DC/DC Controller |
| 中文描述： | 超低噪聲推挽式DC / DC控制器 |
| 文件頁數(shù)： | 11/24頁 |
| 文件大?。?/td> | 237K |
| 代理商： | LT1683 |
11
LT1683
1683f
The voltage slew rate is thus inversely proportional to both
the value of capacitor and R
VSL
.
The current slew feedback loop consists of the voltage
across the external sense resistor, which is internally
amplified and differentiated. The derivative is limited to a
value set by R
CSL
. The current slew rate is thus inversely
proportional to both the value of sense resistor and R
CSL.
The two control loops are combined internally so that a
smooth transition from current slew control to voltage
slew control is obtained. When turning on, the driver
current will slew before voltage. When turning off, voltage
will slew before current. In general it is desirable to have
R
VSL
and R
CSL
of similar value.
Internal Regulator
Most of the control circuitry operates from an internal 2.4V
low dropout regulator that is powered from V
IN
. The
internal low dropout design allows V
IN
to vary from 2.7V
to 20V with stable operation of the controller. When SHDN
< 1.3V the internal regulator is completely disabled.
5V Regulator
A 5V regulator is provided for powering external circuitry.
This regulator draws current from V
IN
and requires V
IN
to
be greater than 6.5V to be in regulation. It can sink or
source 10mA. The output is current limited to prevent
against destruction from accidental short circuits.
Safety and Protection Features
There are several safety and protection features on the
chip. The first is overcurrent limit. Normally the gate
drivers will go low when the output of the internal sense
amplifier exceeds the voltage on the V
C
pin. The V
C
pin is
clamped such that maximum output current is attained
when the CS pin voltage is 0.1V. At that level the outputs
will be immediately turned off (no slew). The effect of this
control is that the output voltage will foldback with
overcurrent.
In addition, if the CS voltage exceeds 0.22V, the V
C
and SS
pins will be discharged to ground also, resetting the soft-
start function. Thus if a short is present this will allow for
faster MOSFET turnoff and less MOSFET stress.
If the voltage on the FB pin exceeds regulation by approxi-
mately 0.22V, the outputs will immediately go low. The
implication is that there is an overvoltage fault.
The voltage on GCL determines two features. The first is
the maximum gate drive voltage. This will protect the
MOSFET gate from overvoltage.
With GCL tied to a Zener or an external voltage source then
the maximum gate driver voltage is approximately
V
GCL
– 0.2V. If GCL is tied to V
IN
, then the maximum gate
voltage is determined by V
IN
and is approximately
V
IN
– 1.6V. There is an internal 19V Zener on the GCL pin
that prevents the gate driver pin from exceeding approxi-
mately 19V.
In addition, the GCL voltage determines undervoltage
lockout of the gate drives. This feature disables the gate
drivers if V
IN
is too low to provide adequate voltage to turn
on the MOSFETs. This is helpful during start up to insure
the MOSFETs have sufficient gate drive to saturate.
If GCL is tied to a voltage source or Zener less than 6.8V,
the gate drivers will not turn on until V
IN
exceeds GCL
voltage by 0.8V. For V
GCL
above 6.5V, the gate drives are
insured to be off for V
IN
< 7.3V and they will be turned on
by V
GCL
+ 0.8V.
If GCL is tied to V
IN
, the gate drivers are always enabled
(undervoltage lockout is disabled).
When driving a push pull transformer, it is important to
make sure that both drivers are not on at the same time.
Even though runaway cannot occur under such cross
conduction with this chip because current slew is regu-
lated, increased current would be possible. This chip has
opposite gate lockout whereby when one MOSFET is on
the other MOSFET cannot be turned on until the gate of the
first drops below 1V. This insures that cross conduction
will not occur.
The gate drives have current limits for the drive currents.
If the sink or source current is greater than 300mA then the
current will be limited.
The V5 regulator also has internal current limiting that will
only guarantee
±
10mA output current.
OPERATIOU
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