NCV891130
www.onsemi.com
11
APPLICATION INFORMATION
Hybrid Low−Power Mode
A high−frequency switch−mode regulator is not very
efficient in light load conditions, making it difficult to
achieve low−Iq requirements for sleep−mode operation. To
remedy this, the NCV891130 includes a low−Iq linear
regulator that turns on at light load, while the PWM
regulator turns off, ensuring a high−efficiency low−power
operation. Another advantage of the low−power mode is the
tight regulation free of voltage ripple usually associated with
low−Iq switchers in light load conditions. In either mode, the
NCV891130 meets the 2% output voltage regulation
specification.
At initial start−up the NCV891130 will soft−start into
PWM converter mode regardless of output current. During
a 300 s period, the NCV891130 will assess the level of
output current. The NCV891130 will not make the
assessment if RSTB is low. If the output current is above the
INtoL threshold, the NCV891130 will stay in PWM mode.
Otherwise, the NCV891130 will transition to low power
mode.
It will stay in this low−power mode until the output current
exceeds the ILIN(lim) limit: it then transitions back to PWM
converter mode. This low−power mode to PWM mode
transition happens within 2 s. The transient response is not
affected by the mode change.
Once the NCV891130 has transitioned to switcher mode,
a 500 s blanking period will occur. After the blanking period,
the NCV891130 will reassess the output current level. If the
output current level is below the INtoL threshold, the
NCV891130 will enter low−Iq mode. If the NCV891130 is
in low−power mode and in normal battery range, it will
transition to switcher mode when VIN increases above
VLINtoSW(HV), regardless of the output current. Similarly, if
the NCV891130 is in PWM mode and VIN is higher than
VFLDUP
, it will not transition to low−power mode even if the
output current becomes lower than INtoL.
At low input voltage, the NCV891130 stays in low−power
mode down to VLINtoSW(LV) if it entered this mode while in
normal battery range. However it may not enter low−power
mode below 8 V depending on the charge of the bootstrap
capacitor (see Bootstrap section for details).
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 NCV891130 may lose regulation and run in
drop−out mode before reaching the UVLO threshold: refer
to the Minimum Vin calculation tool for details. When the
input voltage drops low enough that the part cannot regulate
because it reaches its maximum duty cycle, the switching
frequency is divided down by up to 4 (down to 500 kHz).
This helps lowering the minimum voltage at which the
regulator loses regulation.
An overvoltage monitoring circuit automatically
terminates switching if the input voltage exceeds VOVSTP
(see Figure 29), but the NCV891130 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
VIN Frequency Foldback threshold (see Figure 29).
Frequency reduction is automatically terminated when the
input voltage drops back below the VIN Frequency Foldback
threshold. This also helps to limit the power lost in switching
and generating the drive voltage for the Power Switch.
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3.5 18 20 36 VIN (V)
1
2
FSW
(MHz)
4539
Frequency
folds back
if drop−out
mode
Figure 29. NCV891130 Switching Frequency Profile vs. Input Voltage