NCP1550D ON, NCP1550D Datasheet - Page 14

• Manufacturer Part Number: NCP1550D
• Description: 600 kHz PWM/PFM Step-Down DC-DC Controller
• Manufacturer: ON

Inductor Value Calculation
inductor’s physical size, transient respond
conversion requirements. Lower value inductor saves cost,
PC board space and providing faster transient response, but
result in higher ripple current and core losses. Considering
an application with loading current, I
inductor ripple current, I
than 40% of the load current, i.e. 0.5 A x 40% = 0.2 A.
The relationship between the inductor value and inductor
ripple current is given by,
L +
P−channel MOSFET. Figure 39 is a plot for recommended
inductance against nominal input voltage for different
output options.
P−Channel Power MOSFET Selection
with the NCP1550. The key selection criteria for the power
MOSFET are the gate threshold, V
R
operation, we need to select a low gate threshold device that
can work down to the minimum input voltage level. R
determines the conduction losses for each switching cycle,
the lower the ON resistance, the higher the efficiency can be
achieved. A power MOSFET with lower gate charge can
give lower switching losses but the fast transient can cause
unwanted EMI to the system. Compromise in between is
required during the design stage. For 1.0 A and 2.0 A load
current, NTGS3441T1 and NTGS3443T1 are tested to be
appropriate for most applications.
DS(ON)
Selecting the proper inductance is a trade−off between
Where R
An external P−Channel power MOSFET must be used
12
10
8
6
4
2
0
T ON * (V IN * R DS(ON)
2.2
1.8 V
R
and its total gate charge, Q
DS(ON)
Figure 39. Inductor Selection Chart
DS(ON)
2.7
V
IN
1.9 V
= 0.1 W
I L*RIPPLE(P*P)
, INPUT VOLTAGE OF NCP1550 (V)
is the ON resistance of the external
3.2
2.5 V
L−RIPPLE(P−P)
3.7
2.7 V
I OUT * V OUT)
GS
T
. For low input voltage
, the “ON” resistance,
OUT
is designed to be less
4.2
3.0 V
= 0.5 A and the
APPLICATIONS INFORMATION
3.3 V
and power
4.7
(eq. 1)
DS(ON)
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5.2
NCP1550
14
Flywheel Diode Selection
during the off time. The average diode current depends on
the P−Channel switch duty cycle. At high input voltages, the
diode conducts most of the time. In case of V
V
While the output terminals are shorted, the diode will subject
to its highest stress. Under this condition, the diode must be
able to safely handle the peak current circulating in the loop.
So, it is important to select a flywheel diode that can meet the
diode peak current and average power dissipation
requirements. Under normal conditions, the average current
conducted by the flywheel diode is given by:
forward diode voltage drop.
efficiency. Schottky diodes are a good choice for low
forward drop and fast switching times.
Input and Output Capacitor Selection (C
P−Channel MOSFET is a square wave of duty cycle
(V
a low ESR input capacitor that can support the maximum
RMS input current must be selected. The maximum RMS
input current, I
in below:
where I
simple worst−case condition is used for design.
governed by the required effective series resistance (ESR) of
the capacitor. Typically, once the ESR requirement is met,
the capacitance will be adequate for filtering. The output
voltage ripple, V
effective series resistance of the output capacitor.
ripple contributed by two parts. For most of the case, the
major contributor is the capacitor ESR. Ordinary
aluminum−electrolytic capacitors have high ESR and
should
OUT
I RMS(MAX) [ I OUT
The flywheel diode is turned on and carries load current
Where I
A fast switching diode must also be used to optimize
In continuous mode operation, the source current of the
Above estimation has a maximum value at V
Selection of the output capacitor, C
Where F
From equation (4), it can be noted that the output voltage
OUT
, the diode conducts only a small fraction of the cycle.
+ V
RMS(MAX)
V RIPPLE [ I L * RIPPLE(P*P)
be
D
F
OSC
)/V
is the average diode current and V
RMS(MAX)
avoided.
I D + V IN * V OUT
IN
is the switching frequency and ESR is the
RIPPLE
. To prevent large input voltage transients,
= I
V IN ) V F
OUT
is approximated by:
(ESR )
can be estimated by the equation
V OUT (V IN * V OUT )
Higher
/2. As a general practice, this
4 F OSC C OUT
V IN
quality
I OUT
1
OUT
IN
IN
and C
is primarily
IN
Low
1
2
approaches
)
= 2V
F
(eq. 2)
(eq. 3)
OUT
(eq. 4)
is the
OUT
ESR
)
,