NCP1337 查看數據表（PDF） - ON Semiconductor
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NCP1337 Datasheet PDF : 14 Pages
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NCP1337
The input information is the current injected in FB pin by
the feedback loop. The range of current is from 40 mA for
overload detection to 220 mA corresponding to VCSrippleIN.
In transients, currents from 0 to more than 400 mA may also
appear: the circuitry is able to sustain them.
To regulate the FB pin voltage, the operational amplifier
needs to have a high gain and a wide bandwidth. But the
feedback information used internally needs to be filtered,
because we don’t want the controller to be sensitive to the
switching noise. For this purpose, a 20 kHz filter is added
after the shunt regulator, and any reading of the feedback
signal (for ripple mode, fault detection, or setpoint
elaboration) is done after.
Soft Burst Mode (Protection Mode)
The NCP1337 features a fault timer to detect an overload
completely independently of the VCC voltage. As soon as
the feedback loop asks for the maximum power, a fault is
detected, and an internal timer is started. When the fault
disappears the timer is reset, but if the timer reaches 80 ms,
the protection mode is activated.
Once this protection is toggled, output pulses are stopped
and DSS is deactivated (HV current source turn−on
threshold changes from VCCMIN to VCCLATCH). VCC
slowly decreases (the current consumption is ICC3), and
the HV current source is switched ON when VCC reaches
VCCLATCH. As a result VCC increases until VCCON, but the
controller does not start as the output is still forced low.
VCC decreases again down to VCCLATCH, and a new
start−up cycle occurs. On the second attempt, the output is
released, and NCP1337 effectively starts, with the
soft−start activated. Figure 4 illustrates this behavior.
Safety Features
The NCP1337 includes several safety features to help the
power supply designer to build a rugged design:
• OVP (Overvoltage on VCC): Activated when voltage
on pin VCC is higher than 18.6 V
• Brown−Out (Undervoltage lockout on bulk voltage):
Activated when voltage on pin BO is below 500 mV
• Disable (Comparator activated by an external signal):
Activated when the voltage on BO pin is higher than
3.0 V but below 5.0 V
• TSD (Temperature shutdown): Typically activated
when the die temperature is above 150°C, released at
120°C
All these events have the same consequence for the
controller: the DRV pulses are stopped. When the condition
disappears, the controller restarts with the soft−start
activated.
• Permanent Latch (Comparator activated by an external
signal): Activated when the voltage on BO pin is
above 5.0 V
When this comparator is activated, the DRV pulses are
stopped, and the DSS is deactivated (only the start−up
current source is turned on each time VCC reaches
VCCLATCH, maintaining VCC between 5.0 V and 12 V):
the controller stays in this position until the VCC voltage is
decreased below 4.0 V, i.e., when the power supply is
unplugged from the mains (in normal operation, as soon as
a voltage is present on the HV pin, VCC is always kept
above 5.0 V).
Soft Ripple Mode
The soft ripple mode is a skip mode with a large
hysteresis on the skip comparator in order to ensure a
noise−free and high−efficiency operation in low−load
conditions (standby). When internal setpoint is reaching
VCSrippleIN = 100 mV (corresponding to 20% of the
maximum setpoint), the output pulses are stopped. Then
FB loop asks for more power and internal setpoint is
increasing: when it reaches VCSrippleOUT = 130 mV
(corresponding to 25% of the maximum setpoint), the
output starts pulsing again. Soft−start is activated in each
activity following a stop period. See Figure 5 for detailed
timing diagram.
HV Current Source
NCP1337 features a DSS, to allow operation without any
auxiliary voltage. But to protect the die in case of
short−circuit on VCC pin, the current delivered by the HV
current source is lowered when VCC voltage is below 1.5 V.
In the case the current consumed on the DRV pin is
higher than the DSS capability (high Qg MOSFET or
failure), the HV current source is switched ON when VCC
reaches VCCMIN, but the voltage on VCC pin keep on
decreasing. If there is no UVLO threshold to stop the DRV
pulses, the gate voltage will become too low and the risk is
high to destroy the MOSFET. NCP1337 features an
additional comparator, which threshold is 9.0 V: when VCC
reaches this level whereas the HV current source is ON,
DRV pulses are stopped and the protection mode is
activated.
Brown−Out
The brown−out protection comparator has a fixed
reference of 500 mV. When the comparator is activated
(i.e., when the input voltage VIN is above the starting level),
a 10 mA internal current source is activated and creates an
offset across the bottom resistor of the external resistor
divider. It gives the minimum hysteresis of the brown−out
protection. By adding a series resistor between the divider
and the BO pin, it is possible to adjust (increase) the
hysteresis.
The BO pin also features two additional comparators: the
first one (that toggles at 3.0 V) stops the DRV pulses,
whereas the second one (that toggles at 5.0 V) permanently
latches off the controller (the VCC should be forced below
4.0 V to release the latch).
Figure 8 gives the internal implementation of the BO
pin.
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