IR3895MPBF IRF [International Rectifier], IR3895MPBF Datasheet - Page 28

• Manufacturer Part Number: IR3895MPBF
• Description: 16A HIGHLY INTERGRATED SUPLRBUCK
• Manufacturer: IRF [International Rectifier]

Output Capacitor Selection
The voltage ripple and transient requirements
determine the output capacitors type and values.
The criteria is normally based on the value of the
Effective Series Resistance (ESR). However the actual
capacitance value and the Equivalent Series Inductance
(ESL) are other contributing components.
These components can be described as:
Where:
Since the output capacitor has a major role in the
overall performance of the converter and determines
the result of transient response, selection of the
capacitor is critical. The IR3895 can perform well with
all types of capacitors.
As a rule, the capacitor must have low enough ESR to
meet output ripple and load transient requirements.
The goal for this design is to meet the voltage ripple
requirement in the smallest possible capacitor size.
Therefore it is advisable to select ceramic capacitors
due to their low ESR and ESL and small size. Six of TDK
C2012X5R0J476M (47uF/0805/X5R/6.3V) capacitors is
a good choice.
It is also recommended to use a 0.1µF ceramic
capacitor at the output for high frequency filtering.
ΔV
ΔI
L
= Inductor Ripple Current
0
= Output Voltage Ripple
  
V
o

V
28

o ESL

(
V
V
V
o ESR
o ESR
(
o C
(
)
( )
 
AUGUST 08, 2012 | DATA SHEET| Rev 3.1
)
)




 
V V
8* *
in
L

V
I ESR
C F

L
o ESL
(
o
*
I
o
L



)
*

s
ESL
V
o C
( )

Single‐Input Voltage, Synchronous Buck Regulator
(16)
- 28 -P
16A Highly Integrated SupIRBuck
Feedback Compensation
The IR3895 is a voltage mode controller. The control loop
is a single voltage feedback path including an error
amplifier and error comparator. To achieve fast transient
response and accurate output regulation, a
compensation circuit is necessary. The goal of the
compensation network is to close the control loop at
high crossover frequency with phase margin greater than
45
The output LC filter introduces a double pole, ‐
40dB/decade gain slope above its corner resonant
frequency, and a total phase lag of 180
frequency of the LC filter is expressed as follows:
Figure 25 shows gain and phase of the LC filter. Since we
already have 180
alone,
the system runs the risk of being unstable.
The IR3895 uses a voltage‐type error amplifier with high‐
gain (110dB) and high‐bandwidth (30MHz). The output of
the amplifier is available for DC gain control and AC
phase compensation.
The error amplifier can be compensated either in type II
or type III compensation. Type II compensation is shown
in Fig. 26. This method requires that the output
capacitors have enough ESR to satisfy stability
requirements. If the output capacitor’s ESR generates a
zero at 5kHz to 50kHz, the zero generates acceptable
phase margin and the Type II compensator can be used.
0dB
o
.
Gain
Figure 25: Gain and Phase of LC filter
F
F
LC
LC
-40dB/Decade
Frequency

o
phase shift from the output filter
2


1
L C
o

-180
-90
o
0
0
0
0
Phase
0
0

o
IR3895
F
. The resonant
(17)
LC
PD‐97746
Frequency