NCV8851B ON Semiconductor, NCV8851B Datasheet - Page 9

• Manufacturer Part Number: NCV8851B
• Description: Automotive Grade Synchronous Buck Controller
• Manufacturer: ON Semiconductor

If EN is above 22 V, D
D
125 kW resistor. If D
at least 250 mA is pulled through this diode. The resistor R
must be used if V
Where V
but not yet supplied with 250 mA. For example, setting V
to 1 V means R
at least 250 mA when V
between 22 V and 23 V, D
with the recommended 250 mA current.
UVLO
unexpected behavior does not occur when V
voltage (V
the voltage error amplifier, effectively limiting the slew rate
of V
internal soft−start capacitor based on the internal oscillator,
ZEN1
The EN pin can be tied to V
Undervoltage Lockout (UVLO) is provided to ensure that
Soft−start is achieved by ramping up the internal soft−start
6V
V
V
IN_IC
OUT
OUT
OUT
and
V
Figure 20. Enable Pin Equivalent Structure
. The current to D
IN
t
Z
rising. This ramp is generated by charging an
SS
is the amount of volts where D
Soft−start Delay
), which is applied to the non−inverting input of
V
Threshold
IN
EN
UVLO
EN
R
D
22 V
IN
EN
must be less than 4 kW for D
ZEN2
R
can go above 20 V as follows.
ZEN2
EN(max)
IN
125 kΩ
Figure 21. Normal Start−up
ZEN2
is at least 23 V; for the range of V
is conducting, it is recommended
ZEN2
ZEN1
will be conducting, as well as
+
IN
250 mA
will be conducting, but not
in order to enable the part.
is limited by an internal
V
Z
D
5.4 V
10%*V
ZEN1
ZEN2
Internal
Enable
Soft−start Time
IN_IC
is conducting,
90%*V
OUT
ZEN2
is too low
OUT
to have
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NCV8851B
EN
IN
Z
9
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to support the internal rails and power the controller. The IC
will start up when enabled and V
threshold and will shutdown when V
UVLO threshold minus the UVLO hysteresis. While V
less than the set point for V
duty cycle, after soft−start, once V
UVLO threshold. If EN is high and not tied to V
output will begin to rise up while in UVLO, if a minimum
output load of 1 kW is not met.
Thermal Shutdown
which monitors the die temperature and turns off the top and
bottom gate drivers if an over temperature condition is
detected, for added protection. The internal soft−start
capacitor is also discharged. A normal soft−start will occur
when the die temperature falls below the TSD threshold
minus the TSD hysteresis.
Duty Cycle and Maximum Pulse Width Limits
at an operating point defined by the ratio of the input to the
output voltage. There is a built in minimum off−time which
ensures that the bootstrap supply is charged every cycle,
determining the maximum duty cycle at a given frequency.
while operating at frequencies up to 200 kHz (89% at up to
500 kHz).
Internal Soft−Start
which reduces inrush current and overshoot of the output
voltage. Figures 21 and 22 show a typical soft−start
sequence.
causing the soft−start time to be inversely related to the
frequency set by R
discharged when the part is disabled, enters TSD or enters
UVLO, ensuring a proper start−up when the part is
re−enabled, leaves TSD or leaves UVLO.
The NCV8851B provides Thermal Shutdown (TSD),
In steady state dc operation, the duty cycle will stabilize
The NCV8851B can achieve at least a 95% duty cycle
The NCV8851B features an internal soft−start function,
V
t
Figure 22. Switch−node in Soft−Start
SW
OSC
. The internal soft−start capacitor is
OUT
, the output will run at max
IN_IC
IN_IC
surpasses the UVLO
IN_IC
drops below the
surpasses the
IN
, the
IN
is
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