cs51412g ON Semiconductor, cs51412g Datasheet - Page 13

• Manufacturer Part Number: cs51412g
• Description: 1.5a, 260 Khz And 520 Khz, Low Voltage Buck Regulators With External Bias Or Synchronization Capability
• Manufacturer: ON Semiconductor

using:
where:
with the constant given by Figure 19 at each duty cycle. It is
a common practice to select the input capacitor with an RMS
current rating more than half the maximum load current. If
multiple capacitors are paralleled, the RMS current for each
capacitor should be the total current divided by the number
of capacitors.
Calculated by Multiplying Y Value with Maximum Load
Figure 18. Input Voltage Ripple in a Buck Converter
The RMS current of the input capacitor can be calculated
D = switching duty cycle which is equal to V
I
To calculate the RMS current, multiply the load current
O
0.6
0.5
0.4
0.3
0.2
0.1
0
Figure 19. Input Capacitor RMS Current can be
= load current.
0
0.2
Current at any Duty Cycle
I RMS + I O D(1 * D)
0.4
DUTY CYCLE
CS51411, CS51412, CS51413, CS51414
0.6
0.8
O
/V
IN
http://onsemi.com
.
1.0
13
design's constraint and emphasis. The aluminum
electrolytic capacitors are widely available at lowest cost.
Their ESR and Equivalent Series Inductor (ESL) are
relatively high. Multiple capacitors are usually paralleled to
achieve lower ESR. In addition, electrolytic capacitors
usually need to be paralleled with a ceramic capacitor for
filtering high frequency noises. The OS-CON are solid
aluminum electrolytic capacitors, and therefore has a much
lower ESR. Recently, the price of the OS-CON capacitors
has dropped significantly so that it is now feasible to use
them for some low cost designs. Electrolytic capacitors are
physically large, and not used in applications where the size,
and especially height is the major concern.
Since the ceramic capacitor has low ESR and ESL, a single
ceramic capacitor can be adequate for both low frequency
and high frequency noises. The disadvantage of ceramic
capacitors are their high cost. Solid tantalum capacitors can
have low ESR and small size. However, the reliability of the
tantalum capacitor is always a concern in the application
where the capacitor may experience surge current.
Output Capacitor
capacitor are not as critical as those on the input capacitor.
The current to the output capacitor comes from the inductor
and thus is triangular. In most applications, this makes the
RMS ripple current not an issue in selecting output
capacitors.
caused by ripple current flowing through ESR, and a square
wave due to ESL. Capacitive reactance is assumed to be
small compared to ESR and ESL. The peak-to-peak ripple
current of the inductor is:
to the product of I
across the ESL is proportional to the di/dt of the output
capacitor. It is realized that the di/dt of the output capacitor
is the same as the di/dt of the inductor current. Therefore,
when the switch turns on, the di/dt is equal to (V
and it becomes V
ripple voltage induced by ESL can then be derived from
V
RIPPLE(ESR)
Selecting the capacitor type is determined by each
Ceramic capacitors are now available in values over 10 mF.
In a buck converter, the requirements on the output
The output ripple voltage is the sum of a triangular wave
V
The total output ripple is the sum of the V
V RIPPLE(ESL) + ESL(
RIPPLE(ESR)
.
, the output ripple due to the ESR, is equal
I P * P +
O
/L when the switch turns off. The total
P-P
V IN
and ESR. The voltage developed
L
) ) ESL(
V O (V IN * V O )
(V IN )(L)(f S )
V IN * V O
L
RIPPLE(ESR)
) + ESL(
IN
- V
V IN
L
O
)/L,
and
)