NCP3101 ON Semiconductor, NCP3101 Datasheet - Page 10

• Manufacturer Part Number: NCP3101
• Description: Wide Input Voltage Synchronous Buck Converter
• Manufacturer: ON Semiconductor

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Error = R2 * 1.25 * 10
the desired output voltage as shown in the following
equation:
voltages.
transconductance error amplifier (OTA). The compensation
network consists of the internal error amplifier and the
impedance networks ZIN (R1, R2) and external ZFB
(R
has to provide a closed loop transfer function with the
highest 0 dB crossing frequency to have fast response (but
always lower than f
conditions to minimize the load regulation. A stable control
loop has a gain crossing with -20 dB/decade slope and a
phase margin greater than 45°. Include worst-case
C
SOFT-START
Table 1. OUTPUT VOLTAGES AND DIVIDER
RESISTORS
V
Once R2 is calculated above R3 can be calculated to select
Table 1 shows R3 values for frequently used output
Figure 17 shows a typical Type II operational
comp
OUT
0.8
1.0
1.2
1.5
1.8
2.5
3.3
5.0
, C
(V)
comp
R2
R3
V
R2 (kW
out
Error(%) +
0.51
0.75
and C
R3 +
C
1.8
1.3
1.6
1.6
1.6
2.7
COMP
R
comp
comp
DIS
Figure 17. FB circuit
FB
16
17
)
soft-start
SW
V
-5
V
OUT
REF
R3 (kW) E24
/8) and the highest gain in DC
(%)
R2 * I
V
None
REF
* V
V
0.75
0.51
0.51
2.0
1.5
1.5
1.3
). The compensation network
REF
0.8V
+
bias
-
REF
-
+
UVLO
VCC
POR
* 100
13
* R2
FAULT
R3 (kW) Calculated
2.040
1.500
1.486
1.280
0.753
0.512
0.514
None
(eq. 10)
(eq. 11)
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NCP3101
10
component variations when determining phase margin.
Loop stability is defined by the compensation network
around the OTA, the output capacitor, output inductor and
the output divider. Figure 18 shows the open loop and closed
loop gain plots.
Thermal Considerations
specifications section of this data sheet and a calculation can
be made to determine the NCP3101 junction temperature.
However, it should be noted that the physical layout of the
board, the proximity of other heat sources such as MOSFETs
and inductors, and the amount of metal connected to the
NCP3101, impact the temperature of the device. The PCB
is used also as the heatsink. Double or multi layer PCBs with
thermal vias between places with the same electrical
potential increase cooling area. A 70 mm thick copper
plating is a good solution to eliminate the need for an
external heatsink.
Layout Considerations
layout is very important. Using a good layout can solve
many problems associated with these types of power
supplies as transient occur.
for converter stability. They should be placed close to the
NCP3101. The feedback trace is recommended to be kept as
far from the inductor and noisy power traces as possible. The
resistor divider and feedback acceleration circuit (R2, R3,
R6, C13) is recommended to be placed near to input FB
(Pin 16, NCP3101).
generate voltage transients across the impedances of the
interconnecting bond wires and circuit traces. These
interconnecting impedances should be minimized by using
wide, short printed circuit traces. The critical components
should be located together as close as possible using ground
plane construction or single point grounding. The inductor
and output capacitors should be located together as close as
possible to the NCP3101.
The package thermal resistance can be obtained from the
When designing a high frequency switching converter,
External compensation components (R1, C9) are needed
Switching current from one power device to another can
100
A
Figure 18. Gain Plot for the Error Amplifier
F
Z
Unloaded Gain
1000
Closed Loop,
Gain = GMR
Compensation Network
FREQUENCY (Hz)
Open Loop, Unloaded Gain
10 k
1
F
P
100 k
B
Error Amplifier
1000 k