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NCV51411 Datasheet(PDF) 9 Page - ON Semiconductor

No. de Pieza. NCV51411
Descripción  1.5 A, 260 kHz, Low Voltage Buck Regulator with Synchronization Capability
Descarga  16 Pages
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Fabricante  ONSEMI [ON Semiconductor]
Página de inicio  http://www.onsemi.com
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NCV51411 Datasheet(HTML) 9 Page - ON Semiconductor

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CCOMP = Compensation capacitor connected to the VC pin
ISOURCE = Output Source Current of the error amplifier.
Using a 0.1
mF CCOMP, the calculation shows a TSS over
5.0 ms which is adequate to avoid any current stresses.
Figure 9 shows the gradual rise of the VC, VO and envelope
of the VSW during power up. There is no voltage over−shoot
after the output voltage reaches the regulation. If the supply
voltage rises slower than the VC pin, output voltage may
over−shoot.
Figure 9. The Power Up Transition of NCV51411
Regulator
Short Circuit
When the VFB pin voltage drops below Foldback
Threshold, the regulator reduces the peak current limit by
40% and switching frequency to 1/4 of the nominal
frequency. These features are designed to protect the IC and
external components during over load or short circuit
conditions. In those conditions, peak switching current is
clamped to the current limit threshold. The reduced
switching frequency significantly increases the ripple
current, and thus lowers the DC current. The short circuit can
cause the minimum duty cycle to be limited by Minimum
Output Pulse Width. The foldback frequency reduces the
minimum duty cycle by extending the switching cycle. This
protects the IC from overheating, and also limits the power
that can be transferred to the output. The current limit
foldback effectively reduces the current stress on the
inductor and diode. When the output is shorted, the DC
current of the inductor and diode can approach the current
limit threshold. Therefore, reducing the current limit by
40% can result in an equal percentage drop of the inductor
and diode current. The short circuit waveforms are captured
in Figure 10, and the benefit of the foldback frequency and
current limit is self−evident.
Figure 10. In Short Circuit, the Foldback Current and
Foldback Frequency Limit the Switching Current to
Protect the IC, Inductor and Catch Diode
Thermal Considerations
A calculation of the power dissipation of the IC is always
necessary prior to the adoption of the regulator. The current
drawn by the IC includes quiescent current, pre−driver
current, and power switch base current. The quiescent
current drives the low power circuits in the IC, which
include comparators, error amplifier and other logic blocks.
Therefore, this current is independent of the switching
current and generates power equal to
WQ + VIN
IQ
where:
IQ = quiescent current.
The pre−driver current is used to turn on/off the power
switch and is approximately equal to 12 mA in worst case.
During steady state operation, the IC draws this current from
the Boost pin when the power switch is on and then receives
it from the VIN pin when the switch is off. The pre−driver
current always returns to the VSW pin. Since the pre−driver
current goes out to the regulator’s output even when the
power switch is turned off, a minimum load is required to
prevent overvoltage in light load conditions. If the Boost pin
voltage is equal to VIN + VO when the switch is on, the power
dissipation due to pre−driver current can be calculated by
WDRV + 12 mA
(VIN * VO )
VO2
VIN
)
The base current of a bipolar transistor is equal to collector
current divided by beta of the device. Beta of 60 is used here
to estimate the base current. The Boost pin provides the base
current when the transistor needs to be on. The power
dissipated by the IC due to this current is


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