ISL6455AIRZ-TK Intersil, ISL6455AIRZ-TK Datasheet - Page 11

• Manufacturer Part Number: ISL6455AIRZ-TK
• Description: IC REG TRPL SYNCH DUAL LDO 24QFN
• Manufacturer: Intersil

Specifications of ISL6455AIRZ-TK

• Applications: Converter, FPGAs
• Voltage - Input: 4.2 ~ 5.5 V
• Number Of Outputs: 3
• Voltage - Output: 0.8 ~ 3.3 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-VQFN Exposed Pad, 24-HVQFN, 24-SQFN, 24-DHVQFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ISL6455AIRZ-TKTR

the inductor. The DC resistance of the inductor will influence
directly the efficiency of the converter. Therefore, an inductor
with lowest DC resistance should be selected for highest
efficiency.
In order to avoid saturation of the inductor, the inductor
should be rated at least for the maximum output current plus
the inductor ripple current.
OUTPUT CAPACITOR SELECTION
For the best performance, a low ESR output capacitor is
needed. If an output capacitor is selected with an ESR value
≤120mΩ, its RMS ripple current rating will always meet the
application requirements. The RMS ripple current is
calculated as:
The overall output ripple voltage is the sum of the voltage
spike caused by the output capacitor ESR plus the voltage
ripple caused by charge and discharging the output
capacitor:
Where the highest output voltage ripple occurs at the highest
input voltage.
INPUT CAPACITOR SELECTION
Because of the nature of the buck converter having a
pulsating input current, a low ESR input capacitor is required
for best input voltage filtering and minimizing the
interference with other circuits caused by high input voltage
spikes.
I
∆
RMS C
CURRENT
CAPACITOR
V
OUTPUT
600mA
600mA
O
VALUE
10µF
( )
=
V
O
O
=
TABLE 2. RECOMMENDED CAPACITORS
×
TABLE 1. RECOMMENDED INDUCTORS






INDUCTOR
V
1
-----------------
O
VALUE
L
8.2µH
–
12µH
×
ESR/mΩ
×
V
------- -
V
1
-----------------
f
O
<50
I
L
–






×
V
------- -
×
V
f
O


I
------------------------- -
8 C
Coilcraft
MSS6122-822MX
Coilcraft
MSS6122-123MX
×
×
---------------- -
2
TDK
C2012X5R0J106M
VENDOR PART #
1
×
1
VENDOR PART #
O
11
×
3
f
+
ESR


Ceramic
ISL6455
ISL6455A
COMMENTS
COMMENTS
ISL6455, ISL6455A
The input capacitor should have a minimum value of 10µF
and can be increased without any limit for better input
voltage filtering. The input capacitor should be rated for the
maximum input ripple current calculated as:
The worst case RMS ripple current occurs at D = 0.5.
Ceramic capacitors show good performance because of
their low ESR value, and because they are less sensitive to
voltage transients, compared to tantalum capacitors.
Place the input capacitor as close as possible to the input pin
of the IC for best performance.
Output Voltage Setting
The equations for the Output voltages are given below:
The output resistors should be selected so that the minimum
output load is about 200
Layout Considerations
As for all switching power supplies, the layout is an important
step in the design of ISL6455, ISL6455A based power
supply due to the high switching frequency and low noise
LDO implementations.
Allocate two board levels as ground planes, with many vias
between them to create a low impedance, high-frequency
plane. Tie all the device ground pins through multiple vias
each to this ground plane, as close to the device as possible.
Also tie the exposed pad on the bottom of the device to this
ground plane.
Use wide and short traces for the high current paths. The
input capacitor should be placed as close as possible to the
IC pins as well as the inductor and output capacitor. Use a
common ground node to minimize the effects of ground
noise.
I
VOUT1
VOUT
VOUT2
RMS
=
=
I
=
=
O max
0.45
---------- - Re
(
Rf
1.184
-------------- - Ra
1.184
-------------- - Rc
Rb
Rd
(
)
×
(
(
+
V
------- -
V
Rf
+
+
O
I
Rb
)
Rd
×



µA.
)
)
1
–
V
------- -
V
O
I



December 21, 2005
FN9196.0