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FAN4868 Datasheet(PDF) 9 Page - ON Semiconductor

Part # FAN4868
Description  3 MHz, Synchronous TinyBoost Regulator
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

FAN4868 Datasheet(HTML) 9 Page - ON Semiconductor

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FAN4868
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9
APPLICATION INFORMATION
External Component Selection
Table 8 shows the recommended external components for
the FAN4868:
Table 8. EXTERNAL COMPONENTS
REF
Description
Manufacturer
L1
1.0 μH, 0.8 A,
190 mΩ, 0805
Murata
LQM21PN1R0MC0,
or equivalent
CIN
2.2 μF, 6.3 V, X5R, 0402
Murata
GRM155R60J225M
TDK C1005X5R0J225M
COUT
4.7 μF, 10 V, X5R, 0603
(Note 5)
Kemet
C0603C475K8PAC
TDK C1608X5R1A475K
5. A 6.3 V−rated 0603 capacitor may be used for COUT, such as
Murata GRM188R60J225M. All datasheet parameters are valid
with the 6.3 V−rated capacitor. Due to DC bias effects, the 10 V
capacitor offers a performance enhancement; particularly output
ripple and transient response, without any size increase.
Output Capacitance (COUT)
Stability
The effective capacitance (CEFF) of small, high−value,
ceramic capacitors decrease as their bias voltage increases,
as shown in Figure 18.
Figure 18. CEFF for 4.7 mF, 0603, X5R, 6.3 V
(Murata GRM188R60J475K)
FAN4868 is guaranteed for stable operation with the
minimum value of CEFF (CEFF(MIN)) outlined in Table 9.
Table 9. MINIMUM CEFF REQUIRED FOR STABILITY
Operating Conditions
CEFF(MIN) (mF)
VIN (V)
ILOAD (mA)
2.3 to 4.5
0 to 200
1.5
2.7 to 4.5
0 to 200
1.0
2.3 to 4.5
0 to 150
1.0
CEFF varies with manufacturer, dielectric material, case
size, and temperature. Some manufacturers may be able to
provide an X5R capacitor in 0402 case size that retains CEFF
> 1.5
μF with 5 V bias; others may not. If this CEFF cannot
be economically obtained and 0402 case size is required, the
IC can work with the 0402 capacitor as long as the minimum
VIN is restricted to > 2.7 V.
For best performance, a 10 V−rated 0603 output capacitor
is
recommended
(Kemet
C0603C475K8PAC,
or
equivalent). Since it retains greater CEFF under bias and over
temperature, output ripple can is reduced and transient
capability enhanced.
Output Voltage Ripple
Output voltage ripple is inversely proportional to COUT.
During tON, when the boost switch is on, all load current is
supplied by COUT.
VRIPPLE(P*P) + tON
lLOAD
COUT
(eq. 2)
and
tON + tSW
D + tSW
(1 *
VIN
VOUT
)
(eq. 3)
Therefore:
VRIPPLE(P*P) + tSW (1 *
VIN
VOUT
)
ILOAD
COUT
(eq. 4)
Where:
tSW +
1
fSW
(eq. 5)
As can be seen from Equation 4, the maximum VRIPPLE
occurs when VIN is minimum and ILOAD is maximum.
Startup
Input current limiting is in effect during soft−start, which
limits the current available to charge COUT. If the output
fails to achieve regulation within the time period described
in the soft−start section above; a FAULT occurs, causing the
circuit to shut down, then restart after a significant time
period. If COUT is a very high value, the circuit may not start
on the first attempt, but eventually achieves regulation if no
load is present. If a high−current load and high capacitance
are both present during soft−start, the circuit may fail to
achieve regulation and continually attempt soft−start, only
to have COUT discharged by the load when in the FAULT
state.
The circuit can start with higher values of COUT under full
load if VIN is higher, since:
IOUT + (ILIM(PK) *
IRIPPLE
2
)
VIN
VOUT
(eq. 6)
Generally, the limitation occurs in BST Mode.
The FAN4868 starts on the first pass (without triggering
a FAULT) under the following conditions for CEFF(MAX):


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