IR3897 IRF [International Rectifier], IR3897 Datasheet - Page 28

• Manufacturer Part Number: IR3897
• Description: 4A HIGHLY INTEGRATED 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 IR3897 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. Four of TDK
C2012X5R0J226M (22uF/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

28
V

o ESL
(

V
V
V
o ESR
o ESR
o C
(
(
)
( )
AUGUST 08, 2012 |DATA SHEET | Rev 3.3
 
)


)



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 -
4A Highly Integrated SupIRBuck
Feedback Compensation
The IR3897 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 45o.
The output LC filter introduces a double pole, ‐40dB/decade
gain slope above its corner resonant frequency, and a total
phase lag of 180
expressed as follows:
Figure 25 shows gain and phase of the LC filter. Since we
already have 180
the system runs the risk of being unstable.
The IR3897 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
Gain
F
Figure 25: Gain and Phase of LC filter
F
LC
LC
o
o
. The resonant frequency of the LC filter is

phase shift from the output filter alone,
-40dB/Decade
Frequency
2


1
L C
o

-180
-90
o
TM
0
0
0
0
Phase
0
0

F
LC
(17)
IR3897
Frequency
PD‐97663