NCV8851B ON Semiconductor, NCV8851B Datasheet - Page 12

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

Design Methodology
encompasses the following design process:
(1) Operational Parameter Definition
operational parameters must be defined. These are
application−dependent and include the following:
up−front to use in the design process, as follows:
Where: D
actual duty cycles will be marginally higher than these
calculated values. The actual duty cycles are dependent on
load due to voltage drops in the MOSFETs, inductor and
current sensor.
(2) Switching Frequency Selection
component size and power losses. Operation at higher
switching frequencies allows the use of smaller inductor and
capacitor values to achieve the same inductor current ripple
and output voltage ripple. However, increasing the
frequency increases the switching losses of the MOSFETs,
Choosing external components for the NCV8851B
Before proceeding with the rest of the design, certain
A number of basic calculations must be performed
It should be noted that these are the ideal duty cycles; the
Selecting the switching frequency is a trade−off between
1. Define operational parameters
2. Select switching frequency
3. Select current sensor
4. Select output inductor
5. Select output capacitors
6. Select input capacitors
7. Select compensator components
V
V
I
I
V
D: typical duty cycle (ideal) [%]
V
D
V
OUT
CL
MIN
IN
OUT
IN(max)
IN(typ)
MAX
IN(min)
maximum with a typical value [V]
maximum with initial start−up value [A]
: desired typical current−limit [A]
: input voltage, range from minimum to
: output current, range from minimum to
: minimum duty cycle (ideal) [%]
: output voltage [V]
: maximum duty cycle (ideal) [%]
: typical input voltage [V]
: minimum input voltage [V]
: maximum input voltage [V]
D
D +
D
MIN
MAX
V
+
V
+
IN(typ)
OUT
V
V
V
IN(max)
V
IN(min)
OUT
OUT
APPLICATIONS INFORMATION
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NCV8851B
12
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leading to decreased efficiency, especially noticeable at light
loads.
interfering with signals of known frequencies. Often, in this
case, the frequency can be programmed to a lower value
with R
applied to the SYNC pin to increase the frequency
dynamically to avoid given frequencies. A spread spectrum
signal could also be used for the SYNC input, as long as the
lowest frequency in the range is above the programmed
frequency set by R
frequency must not exceed maximum switching frequency
limits.
frequency: minimum off−time and minimum on−time.
These set two different maximum switching frequencies, as
follows:
Where: F
input voltage can be calculated as follows:
Where: F
resistor connected between the R
grounded side of this resistor should be directly connected
to the AGND pin. Avoid running any noisy signals under the
resistor, since injected noise could cause frequency jitter.
program the frequency. From 150 to 450 kHz, the following
formula is accurate to within 3%:
Where: R
standard 1% resistors can be seen in Table 1.
Typically, the switching frequency is selected to avoid
There are two limits on the maximum allowable switching
Alternatively, the minimum and maximum operational
The switching frequency is programmed by selecting the
The graph in Figure 25 shows the required resistance to
Some specific values for switching frequency with
OSC
T
F
T
SW
SW(max)2
SW(max)1
MinOff
MinOn
OSC
minimum off−time [Hz]
minimum on−time [Hz]
: switching frequency [Hz]
and then a higher−frequency signal can be
: frequency program resistor [W]
V
V
: minimum on−time [s]
: minimum off−time [s]
IN(min)
IN(max)
F
F
: maximum switching frequency due to
: maximum switching frequency due to
SW(max)1
SW(max)2
OSC
R
OSC
+
+
. Additionally, the highest SYNC
1 * T
T
+ 8687000
MinOn
+
+
V
1 * D
T
MinOff
OUT
V
D
F
MinOn
T
@ F
OUT
SW
MIN
OSC
MinOff
SW
MAX
@ F
pin and ground. The
SW
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