IR3856MTRPBF International Rectifier, IR3856MTRPBF Datasheet - Page 19

• 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

Application Information
Design Example:
The following example is a typical application for
IR3856. The application circuit is shown on page
25.
Enabling the IR3856
As explained earlier, the precise threshold of the
Enable lends itself well to implementation of a
UVLO for the Bus Voltage.
For a maximum Enable threshold of V
For a V
good choice.
Programming the frequency
For F
Table. 1.
Output Voltage Programming
Output voltage is programmed by reference
voltage and external voltage divider. The Fb pin
is the inverting input of the error amplifier, which
is internally referenced to 0.7V. The divider is
ratioed to provide 0.7V at the Fb pin when the
output is at its desired value. The output voltage
is defined by using the following equation:
V
Rev2.0
o
V
in
s
=
R
(min)
= 600 kHz, select R
V
in (min)
2
V
V
I
ΔV
F
ref
o
=
o
in
s
*
=
R
=
=
o
=
R
∗
6
1
=10.2V, R
1.8
600
≤
12
1
IR3856
V
⎛
⎜ ⎜
⎝
R
A
+
in(
1
±
2
Enable
V
R
V
+
min
kHz
5%
2
V
(
R
R
13.2V
EN
=
)
8
9
of
−
V
V
1
⎞
⎟ ⎟
⎠
EN
V
=49.9K and R
EN
o
max)
..........
=
t
..........
= 23.7 kΩ, using
1.36V.....
(7)
..........
V
......
2
EN
in
=7.5K is a
R
R
= 1.36 V
(6)
1
2
.(8)
When an external resistor divider is connected to
the output as shown in figure 12.
Equation (6) can be rewritten as:
For the calculated values of R8 and R9 see
feedback compensation section.
Soft-Start Programming
The soft-start timing can be programmed by
selecting the soft-start capacitance value. From
(1), for a desired start-up time of the converter,
the soft start capacitor can be calculated by
using:
Where T
For a start-up time of 3.5ms, the soft-start
capacitor will be 0.099μF. Choose a 0.1μF
ceramic capacitor.
Bootstrap Capacitor Selection
To drive the Control FET, it is necessary to
supply a gate voltage at least 4V greater than
the voltage at the SW pin, which is connected
the source of the Control FET . This is achieved
by using a bootstrap configuration, which
comprises the internal bootstrap diode and an
external bootstrap capacitor (C6), as shown in
Fig. 13. The operation of the circuit is as follows:
When the lower MOSFET is turned on, the
capacitor node connected to SW is pulled down
to ground. The capacitor charges towards V
through the internal bootstrap diode, which has a
forward voltage drop V
the bootstrap capacitor C6 is approximately
given as
When the upper MOSFET turns on in the next
cycle, the capacitor node connected to SW rises
to the bus voltage V
C6 is appropriately chosen, the voltage Vc
Fig. 12. Typical application of the IR3856 for
C
R
SS
9
=
(
V
start
R
μF
programming the output voltage
c
IR3856
8
IR3624
≅
∗
)
is the desired start-up time (ms).
⎛
⎜ ⎜
⎝
V
=
V
cc
T
V
o
start
−
−
ref
V
V
ref
Fb
D
(
⎞
⎟ ⎟
⎠
ms
in
..........
. However, if the value of
..........
IR3856MPbF
D
. The voltage V
)
×
..........
0.02857
..........
V
OUT
......
..........
R
R
PD-97528
9
8
..........
(11)
....
c
(9)
across
(10)
19
cc