NCV890130
http://onsemi.com
11
GENERAL INFORMATION
INPUT VOLTAGE
An Undervoltage Lockout (UVLO) circuit monitors the
input, and inhibits switching and resets the Soft−start circuit
if there is insufficient voltage for proper regulation. The
NCV890130 can regulate a 3.3 V output with input voltages
above 4.5 V and a 5.0 V output with an input above 6.5 V.
The NCV890130 automatically terminates switching if
input voltage exceeds V
OVSTP
(see Figure 25), and
withstands input voltages up to 45 V.
To limit the power lost in generating the drive voltage for
the Power Switch, the switching frequency is reduced by a
factor of 2 when the input voltage exceeds the V
IN
Frequency Foldback threshold V
FLDUP
(see Figure 25).
Frequency reduction is automatically terminated when the
input voltage drops back below the V
IN
Frequency Foldback
threshold V
FLDDN
.
4
18 20 36
VIN (V)
1
2
45
30
Figure 25. NCV890130 Switching Frequency
Reduction at High Input Voltage
Fsw
(MHz)
ENABLE
The NCV890130 is designed to accept either a logic level
signal or battery voltage as an Enable signal. EN low induces
a ’sleep mode’ which shuts off the regulator and minimizes
its supply current to a couple of mA typically (I
qSD
) by
disabling all functions. Upon enabling, voltage is
established at the DRV pin, followed by a soft−start of the
switching regulator output.
SOFT−START
Upon being enabled or released from a fault condition,
and after the DRV voltage is established, a soft−start circuit
ramps the switching regulator error amplifier reference
voltage to the final value. During soft−start, the average
switching frequency is lower than its normal mode value
(typically 2 MHz) 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. For 3.3 V and 5 V output
voltages, the recommended inductor value is 4.7 mH.
SHORT CIRCUIT FREQUENCY FOLDBACK
During severe output overloads or short circuits, the
NCV890130 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. If the current is still too high after the
switching frequency folds back to 500 kHz, the regulator
enters an auto−recovery burst mode that further reduces the
dissipated power.
CURRENT LIMITING
Due to the ripple on the inductor current, the average
output current of a buck converter is lower than the peak
current setpoint of the regulator. Figure 26 shows − for a
4.7 mH inductor − how the variation of inductor peak current
with input voltage affects the maximum DC current the
NCV890130 can deliver to a load.
Figure 26. NCV890130 Load Current Capability
with 4.7 mH Inductor
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
0 5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
MINIMUM CURRENT LIMIT (A)
(5 V
OUT
)
(3.3 V
OUT
)
