FAN4860 Datasheet, PDF (11/15 Pages) Fairchild Semiconductor – 3MHz, 5V Output Synchronous TinyBoost™ Regulator

Application Information
External Component Selection
Table 2 shows the recommended external components for
the FAN4860:
Table 2. External Components
REF Description
Manufacturer
L1
1.0µH, 0.8A, 190mΩ, Murata LQM21PN1R0MC0,
0805
or equivalent
CIN
2.2µF, 6.3V, X5R,
0402
Murata GRM155R60J225M
TDK C1005X5R0J225M
COUT
4.7µF, 10V, X5R,
0603⁽³⁾
Kemet C0603C475K8PAC
TDK C1608X5R1A475K
Note:
3. A 6.3V-rated 0603 capacitor may be used for COUT,
such as Murata GRM188R60J225M. All datasheet
parameters are valid with the 6.3V-rated capacitor.
Due to DC bias effects, the 10V capacitor offers a
performance enhancement; particularly output ripple
and transient response, without any size increase.
Output Capacitance (C_OUT)
Stability
The effective capacitance (CEFF) of small, high-value,
ceramic capacitors decrease as their bias voltage increases,
as shown in Figure 30.
Figure 30. CEFF for 4.7μF, 0603, X5R, 6.3V
(Murata GRM188R60J475K)
FAN4860 is guaranteed for stable operation with the
minimum value of CEFF (CEFF(MIN)) outlined in Table 3.
Table 3. Minimum C_EFF Required for Stability
Operating Conditions
V_IN (V)
I_LOAD (mA)
C_EFF(MIN) (μF)
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
>1.5μF with 5V 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 >2.7V.
For best performance, a 10V-rated 0603 output capacitor is
recommended (Kemet C0603C475K8PAC, or equivalent).
Since it retains greater CEFF under bias and over
temperature, ouptut 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.
V_{RIPPLE(P −P)}
=
t _ON
•
I_LOAD
C_OUT
and
EQ. 2
t _ON
=
t _SW
•D
=
t _SW
• _{⎜⎜⎝⎛}1−
V_IN
V_OUT
⎟⎟⎠⎞
Therefore:
EQ. 3
V_{RIPPLE(P −P)}
=
t _SW
• _{⎜⎜⎝⎛}1−
V_IN
V_OUT
⎟⎟⎠⎞ ^•
I_LOAD
C_OUT
EQ. 4
where:
t_SW
=
1
f_SW
EQ. 5
As can be seen from EQ. 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
© 2009 Fairchild Semiconductor Corporation
FAN4860 • Rev. 1.0.3
11
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