IR3856MTRPBF International Rectifier, IR3856MTRPBF Datasheet - Page 21

• Manufacturer Part Number: IR3856MTRPBF
• Description: IC BUCK SYNC ADJ 6A 17PQFN
• Manufacturer: International Rectifier
• Series: SupIRBuck™r
• Type: Step-Down (Buck)r

Specifications of IR3856MTRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.7 ~ 18.9 V
• Current - Output: 6A
• Frequency - Switching: 250kHz ~ 1.5MHz
• Voltage - Input: 1.5 ~ 21 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 17-PowerVQFN
• Power - Output: 1.11W
• Primary Input Voltage: 21V
• No. Of Outputs: 1
• Output Voltage: 18.9V
• Output Current: 6A
• No. Of Pins: 17
• Operating Temperature Range: -40°C To +125°C
• Peak Reflow Compatible (260 C): Yes
• Rohs Compliant: Yes
• Leaded Process Compatible: Yes
• Part Status: Preferred
• Package: PQFN / 4 x 5
• Circuit: Single Output
• Iout (a): 6
• Switch Freq (khz): 250 - 1500
• Input Range (v): 1.5 - 21
• Output Range (v): 0.7 - 0.9*Vin
• Ocp Otp Uvlo Pre-bias Soft Start And: PGOOD + EN + SEQ + OVP + SYNC
• Design Tool: Yes
• Server Storage: Yes
• Routers Switches: Yes
• Base Station Telecom: Yes
• Digital Home Media: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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.
described as
Feedback Compensation
The IR3856 is a voltage mode controller. The
control loop is a single voltage feedback path
including 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 provide a closed-loop transfer
function with the highest 0 dB crossing frequency
and adequate phase margin (greater than 45
Since the output capacitor has a major role in the
overall
determines the result of transient response,
selection of the capacitor is critical. The IR3856
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
C2102X5R0J226M
capacitors is a good choice.
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Rev2.0
V
Δ
Δ
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I
V
L
Δ
=
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V
=
small
Δ
=
Δ
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Inductor
V
Δ
performance
V
Output
ESL
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V
( o
( o
ESR
ESR
)
C
=
)
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⎜
⎝
=
size.
+
V
=
8
in
Δ
voltage
ripple
Δ
*
V
L
−
I
C
( o
L
Δ
V
(22uF,
o
*
I
ESL
o
L
*
ESR
of
Four
⎞
⎟
⎠
components
F
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)
s
current
ESL
+
ripple
the
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6.3V,
of
C
converter and
)
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the
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TDK
).
(16)
be
The output LC filter introduces a double pole,
–40dB/decade gain slope above its corner
resonant frequency, and a total phase lag of 180
(see figure 13). The resonant frequency of the LC
filter is expressed as follows:
Figure 14 shows gain and phase of the LC filter.
Since we already have 180
output filter alone, the system runs the risk of
being unstable.
The IR3856 uses a voltage-type error amplifier
with high-gain (110dB) and wide-bandwidth. 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.
Local feedback with Type-II compensation is
shown in figure 14.
This method requires that the output capacitor
should have enough ESR to satisfy stability
requirements.
compensation
generates a zero typically at 5kHz to 50kHz
which is essential for an acceptable phase
margin.
The ESR zero of the output capacitor is
expressed as follows:
0 dB
F
Gain
ESR
F
LC
Fig. 14. Gain and Phase of LC filter
=
=
2
F
2
LC
∗
∗
π
π
-40dB/decade
Frequency
*ESR*C
1
L
1
o
the
In
∗
C
o
output
IR3856MPbF
o
general,
..........
-180
0
..........
Phase
0
o
0
phase shift from the
..........
capacitor’s
F
..........
LC
PD-97528
for
..........
Frequency
.......
Type-II
..
(17)
21
ESR
(18)
o