ncp3170 ON Semiconductor, ncp3170 Datasheet - Page 23

• Manufacturer Part Number: ncp3170
• Description: Ncp3170 Synchronous Pwm Switching Converter
• Manufacturer: ON Semiconductor

lockout, turns on at a defined voltage level, and draws a
constant current, then the RMS connected load current is:
I
V
V
current during max load, then an input fuse should be rated
accordingly using I
OUT
OUT
OUT_TO
Alternatively, if the output load has an under voltage
If the inrush current is higher than the steady state input
Figure 50. Voltage Enable Load Current
492 mA +
Figure 49. Resistive Load Current
I
CL1
= Output current
= Output voltage
= Output voltage load turn on
Voltage
Current
Output
Output
+
2
t methodology.
1.0 V
3.3 V * 2.5 V
V
OUT
3.3 V
tss
tss
* V
V
OUT
Voltage
Current
Output
Output
OUT_TO
t
1 A
I
3.3 V
OUT
3.3. V
(eq. 50)
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23
Thermal Management and Layout
Consideration
through two loops as shown in the figure below.
switch turns on. When the switch turns on, the edge of the
current waveform is provided by the bypass capacitor. The
remainder of the current is provided by the input capacitor.
Slower currents are provided by the upstream power supply
which fills up the input capacitor when the high side switch
is off. The current flows through the high side MOSFET and
to the output, charging the output capacitors and providing
current to the load. The current returns through a PCB
ground trace where the output capacitors are connected, the
regulator is grounded, and the input capacitors are grounded.
The second loop starts from the inductor to the output
capacitors and load, and returns through the low side
MOSFET. Current flows in the second loop when the low
side NMOSFET is on. The designer should note that there
are locations where the red line and the blue line overlap;
these areas are considered to have DC current. Areas
containing a single blue line indicate that AC currents flow
and transition very quickly. The key to power supply layout
is to focus on the connections where the AC current flows.
20nH of inductance exists. When laying out a PCB,
minimizing the AC loop area reduces the noise of the circuit
and improves efficiency. A ground plane is strongly
recommended to connect the input capacitor, output
capacitor, and PGND pin of the NCP3170. Drawing the real
high power current flow lines on the recommended layout is
important so the designer can see where the currents are
flowing.
In the NCP3170 buck regulator high pulsing current flows
22 mF
The first loop shown in blue activates when the high side
A good rule of thumb is that for every inch of PCB trace,
C1
Figure 51. Buck Converter Current Paths
Current
C
0.1 mF
VIN
bypass
Input
C
c
R
c
EN
PG
COMP
AGND
DRIVE
VIN
PGND
VSW
FB1
L1 4.7 mH
R1
R2
C2,
C3
22 mF
3.3 V