NCV890430
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7
APPLICATION INFORMATION
General Description
The NCV890430 is a high-frequency synchronous
switch-mode regulator with current-mode control, fixed
output voltage and fixed internal closed-loop compensation,
accepting a wide input voltage range typical to automotive
applications.
The use of a P-channel high-side MOSFET simplifies the
driving scheme (no bootstrap circuitry needed), and enables
a duty cycle of 100% for low dropout operation at low input
voltage.
Input Voltage
An Undervoltage Lockout (UVLO) circuit monitors the
input, and can inhibit switching and reset the soft-start
circuit if there is insufficient voltage for proper regulation.
Depending on the output conditions (voltage option and
loading), the NCV890430 may lose regulation and run in
drop-out mode before reaching the UVLO threshold: refer
to the Minimum V
IN
calculation tool for details. When the
input voltage drops low enough that the part cannot regulate
because it reaches its maximum duty cycle, the high-side
MOSFET can turn on permanently (100% duty cycle
operation), to help lower the minimum voltage at which the
regulator loses regulation.
An overvoltage monitoring circuit automatically
terminates switching and disables the output if the input
voltage exceeds 37 V (see Figure 9
), but the NCV890430
can withstand input voltages up to 45 V.
To avoid skipping switching pulses and entering an
uncontrolled mode of operation, the switching frequency is
reduced by a factor of 2 when the input voltage exceeds the
V
IN
Frequency Foldback threshold (see Figure 9 below).
Frequency reduction is automatically terminated when the
input voltage drops back below the V
IN
Frequency Foldback
threshold. This also helps to limit the power lost in switching
and generating the drive voltage for the Power Switches at
high input voltage. Above the frequency foldback threshold,
improved efficiency can be expected due to the lower
switching frequency.
Figure 9. NCV890430 Worst-Case Switching Frequency Profile vs. Input Voltage
100% duty
cycle if
drop−out
mode
2
1
3.5 18 20 34.5 40 45
V
IN
(V)
f
SW
(MHz)
4.5
Soft-Start
Upon being enabled or released from a fault condition, a
soft-start circuit ramps the switching regulator error
amplifier reference voltage to the final value, forcing the
output to follow the same soft-start ramp. During soft-start,
the average switching frequency is lower until the output
voltage approaches regulation.
Slope Compensation
A fixed slope compensation signal is generated internally
and added to the sensed current to avoid increased output
voltage ripple due to bifurcation of inductor ripple current
at duty cycles above 50%. The fixed amplitude of the slope
compensation signal requires the inductor to be greater than
a minimum value, depending on output voltage, in order to
avoid sub-harmonic oscillations. The recommended
inductor value is between 1.8 and 3.3 mH, although other
values are possible.
Short Circuit Protection
During severe output overloads or short circuits, the
NCV890430 automatically reduces its switching frequency.
This creates duty cycles small enough to limit the peak
current in the power components, while maintaining the
ability to automatically reestablish the output voltage if the
overload is removed.
In more severe short-circuit conditions where the inductor
current reaches the peak current limit during the minimum
on time, the regulator enters a hiccup mode that further
reduces the power dissipation and protects the system.
