ncp3155a ON Semiconductor, ncp3155a Datasheet - Page 15

• Manufacturer Part Number: ncp3155a
• Description: Ncp3155a, Ncp3155b 3 A Synchronous Buck Regulator
• Manufacturer: ON Semiconductor

the low side MOSFET is off for the majority of the switching
period. Reduced sampling time causes errors in the
regulated voltage on the boost pin. High duty cycle / input
voltage induced sampling errors can result in increased
boost ripple voltage or higher than desired DC boost voltage.
Figure 38 outlines all operating regions.
Region 1 (Green) where a 0.1 mF, 25 V ceramic capacitor
can be placed on the boost pin without causing damage to the
device or MOSFETS. Larger boost ripple voltage occurring
Reduced sampling time occurs at high duty cycles where
The recommended operating conditions are shown in
24
22
20
18
16
12
10
14
8
6
4
2
Normal Operation
5
(Region 1)
10
Figure 38. Safe Operating Area for Boost Voltage with a 0.1 mF Capacitor
15
20
25
Region 1
30
BOOST VOLTAGE LEVELS
35
DUTY CYCLE
Increased Boost Ripple
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(Still in Specification)
11.5V
40
22V
(Region 2)
45
15
over several switching cycles is shown in Region 2 (Yellow).
The boost ripple frequency is dependent on the output
capacitance selected. The ripple voltage will not damage the
device or $12 V gate rated MOSFETs.
damage the device or $12 V gate rated MOSFETs can be
seen in Region 3 (Orange). Placing a boost capacitor that is
no greater than 3.3 nF on the boost pin limits the maximum
boost voltage < 12 V. The typical drive waveforms for
Regions 1, 2 and 3 (green, yellow, and orange) regions of
Figure 38 are shown in Figure 39.
50
Conditions where maximum boost ripple voltage could
55
60
65
70
71%
Increased Boost Ripple
Capacitor Optimization
Region 2
Required (Region 3)
Region 3
75
80
Cycle
85
Duty
Max
90