NCV887200
www.onsemi.com
9
Internal Soft−Start
To insure moderate inrush current and reduce output
overshoot, the NCV887200 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 NCV887200 is synchronized to
twice the default switching frequency the soft start will last
half as long.
VDRV
An internal regulator provides the drive voltage for the
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.
GDRV
An R
GND
= 15 kW (typical) GDRV−GND resistor is
strongly recommended.
APPLICATION INFORMATION
Design Methodology
This section details an overview of the component selection
process for the NCV8872 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
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
10. Determine Feedback Loop Compensation Network
1. Define Operational Parameters
Before beginning the design, define the operating
parameters of the application. These include:
V
IN(min)
: minimum input voltage [V]
V
IN(max):
maximum input voltage [V]
V
OUT
: output voltage [V]
I
OUT(max)
: maximum output current [A]
I
CL
: desired typical cycle-by-cycle current limit [A]
From this the ideal minimum and maximum duty cycles
can be calculated as follows:
D
min
+ 1 *
V
IN(max)
V
OUT
D
max
+ 1 *
V
IN(min)
V
OUT
Both duty cycles will actually be higher due to power loss
in the conversion. The exact duty cycles will depend on
conduction and switching losses. If the maximum input
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.
If the calculated D
max
is higher the D
max
of the
NCV887200, the conversion will not be possible. It is
important for a boost converter to have a restricted D
max
,
because while the ideal 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.
If the following equation is not satisfied, the device will
skip pulses at high V
IN
:
D
min
f
s
w t
on(min)
Where: f
s
: switching frequency [Hz]
t
on(min)
: minimum on time [s]
2. Select Current Sense Resistor
Current sensing for peak current mode control and current
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:
R
S
+
V
CL
I
CL
Where: R
S
: sense resistor [W]
V
CL
: current limit threshold voltage [V]
I
CL
: desire current limit [A]
3. Select Output Inductor
The output inductor controls the current ripple that occurs
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 20−40% of the inductor
current at the maximum load at the worst case V
IN
, but
operation should be verified empirically. The worst case V
IN
is half of V
OUT
, or whatever V
IN
is closest to half of V
OUT
.
After choosing a peak current ripple value, calculate the
inductor value as follows:
L +
V
IN(WC)
D
WC
DI
L,max
f
s
