IR3856MTRPBF International Rectifier, IR3856MTRPBF Datasheet - Page 23

• 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

The compensation network has three poles and
two zeros and they are expressed as follows:
Cross over frequency is expressed as:
F
Based on the frequency of the zero generated by
the output capacitor and its ESR, relative to
crossover frequency, the compensation type can
be different. The table below shows the
compensation types for relative locations of the
crossover frequency.
F
F
F
F
F
H(s) dB
o
P
P
P
Z
Z
Rev2.0
Fig.16. Type III Compensation network and
its asymptotic gain plot
1
1
2
3
2
=
=
=
=
=
=
R
0
Z
2
2
2
2
3
π
IN
..........
π
π
π
*
Gain(dB)
*
C
*
*
*
R
R
C
C
R
R
7
1
10
1
3
*
7
10
7
3
F
*
V
⎛
⎜ ⎜
⎝
( *
..........
V
V
Z
1
*
C
C
osc
C
1
1
in
OUT
C
R
4
4
4
R
7
R
8
+
..........
*
*
8
..........
9
+
C
C
2
..........
F
R
π
3
3
Z
V
2
10
⎞
⎟ ⎟
⎠
*
Fb
REF
..........
L
≅
)
1
..........
o
≅
..........
2
*
π
C
2
R
π
*
o
E/A
..........
F
3
R
*
..........
..........
P
1
..........
3
C
2
1
*
7
C
C
*
3
.......
3
R
..........
..........
..........
8
F
..........
C
P
..........
3
Comp
4
........(3
Frequency
...
...(28
..
..(27)
(32)
(29)
(31)
Z
Ve
f
0)
)
The
potentially faster the load transient response.
However, the crossover frequency should be low
enough to allow attenuation of switching noise.
Typically, the control loop bandwidth or crossover
frequency is selected such that
The DC gain should be large enough to provide
high DC-regulation accuracy. The phase margin
should be greater than 45
For this design we have:
V
V
V
V
L
C
It must be noted here that the value of the
capacitance used in the compensator design
must be the small signal value. For instance, the
small signal capacitance of the 22uF capacitor
used in this design (i.e. C3216X5R1E106M from
TDK) is 9.5uF at 1.8 V DC bias and 600 kHz
frequency. It is this value that must be used for all
computations related to the compensation. The
small signal value may be obtained from the
manufacturer’s
SPICE models. Alternatively, they may also be
inferred from measuring the power stage transfer
function of the converter and measuring the
double pole frequency F
(16) to compute the small signal C
These result to:
F
F
F
Select crossover frequency F
Since F
place the pole and zeros.
o
LC
ESR
s
in
o
osc
ref
o
=1.0uH
/2=300 kHz
=1.8V
=4x22uF, ESR=3mOhm each
=12V
=0.7V
=25.8 kHz
=1.8V
=5.5 MHz
Compensator
Compensator
Type
Type
higher
LC
Type III
Type III
Type II
Type II
<F
o
<F
F
the
o
s
/2<F
datasheets,
≤
F
F
(
1/5
crossover
LC
LC
ESR
F
F
IR3856MPbF
<F
<F
F
F
LC
LC
~
ESR
ESR
, Type-III is selected to
LC
o
<F
<F
ESR
ESR
1/10
for overall stability.
o
o
vs F
vs F
and using equation
<F
<F
<F
<F
o
=100 kHz
)
o
o
design
ESR
ESR
<F
<F
*
o
o
F
frequency,
o
s
s
s
PD-97528
/2
/2
.
Electrolytic
Electrolytic
Tantalum
Tantalum
Output
Output
Capacitor
Capacitor
Tantalum
Tantalum
Ceramic
Ceramic
tools or
23
the