NCV8871LVBGEVB ON Semiconductor, NCV8871LVBGEVB Datasheet - Page 10

• Manufacturer Part Number: NCV8871LVBGEVB
• Description: Power Management IC Development Tools NCV8871 EVAL BOARD
• Manufacturer: ON Semiconductor

Specifications of NCV8871LVBGEVB

• Rohs: yes

and will shut down when VIN drops below the UVLO
threshold or the part is disabled.
Internal Soft−Start
overshoot, the NCV8871 features a soft start which charges a
capacitor with a fixed current to ramp up the reference voltage.
This fixed current is based on the switching frequency, so
that if the NCV8871 is synchronized to twice the default
switching frequency the soft start will last half as long.
VDRV
gate driver. Bypass with a ceramic capacitor to ground to
ensure fast turn on times. The capacitor should be between
0.1 mF and 1 mF, depending on switching speed and charge
requirements of the external MOSFET.
APPLICATION INFORMATION
Design Methodology
process for the NCV8871 in continuous conduction mode
boost. It is intended to assist with the design process but does
not remove all engineering design work. Many of the
equations make heavy use of the small ripple approximation.
This process entails the following steps:
1. Define Operational Parameters
parameters of the application. These include:
can be calculated as follows:
in the conversion. The exact duty cycles will depend on
conduction and switching losses. If the maximum input
To insure moderate inrush current and reduce output
An internal regulator provides the drive voltage for the
This section details an overview of the component selection
1. Define Operational Parameters
2. Select Current Sense Resistor
3. Select Output Inductor
4. Select Output Capacitors
5. Select Input Capacitors
6. Select Feedback Resistors
7. Select Compensator Components
8. Select MOSFET(s)
9. Select Diode
Before beginning the design, define the operating
V
V
V
I
I
From this the ideal minimum and maximum duty cycles
Both duty cycles will actually be higher due to power loss
OUT(max)
CL
IN(min)
IN(max):
OUT
: desired typical cycle-by-cycle current limit [A]
: output voltage [V]
: minimum input voltage [V]
maximum input voltage [V]
: maximum output current [A]
D
D
min
max
+ 1 *
+ 1 *
V
V
V
V
IN(max)
IN(min)
OUT
OUT
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voltage is higher than the output voltage, the minimum duty
cycle will be negative. This is because a boost converter
cannot have an output lower than the input. In situations
where the input is higher than the output, the output will
follow the input, minus the diode drop of the output diode
and the converter will not attempt to switch.
the conversion will not be possible. It is important for a boost
converter to have a restricted D
conversion ration of a boost converter goes up to infinity as
D approaches 1, a real converter’s conversion ratio starts to
decrease as losses overtake the increased power transfer. If
the converter is in this range it will not be able to regulate
properly.
skip pulses at high V
2. Select Current Sense Resistor
limit relies on the MOSFET current signal, which is
measured with a ground referenced amplifier. The easiest
method of generating this signal is to use a current sense
resistor from the source of the MOSFET to device ground.
The sense resistor should be selected as follows:
3. Select Output Inductor
over a switching period. A high current ripple will result in
excessive power loss and ripple current requirements. A low
current ripple will result in a poor control signal and a slow
current slew rate in case of load steps. A good starting point
for peak to peak ripple is around 10% of the inductor current
at the maximum load at the worst case V
should be verified empirically. The worst case V
V
choosing a peak current ripple value, calculate the inductor
value as follows:
OUT
If the calculated D
If the following equation is not satisfied, the device will
Current sensing for peak current mode control and current
The output inductor controls the current ripple that occurs
Where: f
Where: R
Where: V
, or whatever V
t
V
I
half of V
D
DI
on(min)
CL
CL
WC
L,max
s
: desire current limit [A]
: switching frequency [Hz]
S
IN(WC)
: current limit threshold voltage [V]
: sense resistor [W]
: duty cycle at V
: minimum on time [s]
: maximum peak to peak ripple [A]
L +
OUT
max
IN
: V
D
IN
:
f
min
is higher the D
s
R
DI
IN
[V]
V
S
is closest to half of V
IN(WC)
L,max
value as close as possible to
w t
+
V
on(min)
I
max
CL
CL
IN(WC)
f
2
s
D
V
, because while the ideal
OUT
WC
max
IN
of the NCV8871,
, but operation
IN
IN
is half of
. After