ISL6455 Datasheet, PDF (11/13 Pages) Intersil Corporation – Low shutdown supply current

ISL6455, ISL6455A
capacitor must be located at a distance of not more than 0.5
inches from the V_IN pins of the IC and returned to a clean
analog ground. Any good quality ceramic capacitor can be
used as an input capacitor.
The output capacitor must meet the requirements of
minimum amount of capacitance and ESR for both LDOs.
The ISL6455 is specifically designed to work with small
ceramic output capacitors. The output capacitor’s ESR
affects stability and output noise. Use an output capacitor
with an ESR of 50m or less to insure stability and optimum
transient response. For stable operation, a ceramic
capacitor, with a minimum value of 3.3µF, is recommended
for V_OUT1 for 300mA output current, and 3.3µF is
recommended for V_OUT2 at 300mA load current. There is no
upper limit to the output capacitor value. A larger capacitor
can reduce noise and improve load transient response,
stability and PSRR. A higher value output capacitor (10µF) is
recommended for LDO2 when used to power VCO circuitry
in wireless chipsets. The output capacitor should be located
very close to V_OUT pins to minimize impact of PC board
inductances and the other end of the capacitor should be
returned to a clean analog ground.
PWM Regulator Component Selection
INDUCTOR SELECTION
A 8.2µH typical output inductor is used with the ISL6455 and
a 12µH typical with the ISL6455A PWM section. Values less
than this may cause stability problems because of the
internal compensation of the regulator. The important
parameters of the inductor that need to be considered are
the current rating of the inductor and the DC resistance of
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. (See Table 1).
TABLE 1. RECOMMENDED INDUCTORS
OUTPUT INDUCTOR
CURRENT VALUE
VENDOR PART
NUMBER
COMMENTS
600mA
8.2µH Coilcraft
MSS6122-822MX
ISL6455
600mA
12µH
Coilcraft
MSS6122-123MX
ISL6455A
OUTPUT CAPACITOR SELECTION
For the best performance, a low ESR output capacitor is
needed. If an output capacitor is selected with an ESR value
120m, its RMS ripple current rating will always meet the
application requirements. The RMS ripple current is
calculated as shown in Equation 1:
I_{RMSCO}
=
V_O

1_{----L-–-----}^V---_V----f--^O_---I--

-------¹---------
2 3
(EQ. 1)
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 as shown in Equation 2:
V_O
=
V_O





-¹---_L-–----_-^V---V----_f--^O---^I--_{}





8^-----^-----C^--1---_O^-----^-----f
+
E S R_


(EQ. 2)
Where the highest output voltage ripple occurs at the highest
input voltage.
TABLE 2. RECOMMENDED CAPACITORS
CAPACITOR
VALUE
ESR/m
VENDOR PART
NUMBER
COMMENTS
10µF
<50 TDK
Ceramic
C2012X5R0J106M
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.
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 shown in
Equation 3:
I_RMS = I_{Omax} 
V_--V---O-I-




1
–
V_{--V---O-I-}
(EQ. 3)
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.