FAN2315MPX Datasheet, PDF (13/19 Pages) Fairchild Semiconductor – FAN2315 TinyBuck 15 A Integrated Synchronous Buck Regulator

Application Information
Stability
Constant on-time stability consists of two parameters:
stability criterion and sufficient signal at V_FB.
Stability criterion is given by:
×
≫2
(7)
Sufficient signal requirement is given by:
∆×
>∆
(8)
where ΔI_IND is the inductor current ripple and ΔV_FB is
the ripple voltage on V_FB, which should be ≥12 mV.
In certain applications, especially designs utilizing only
ceramic output capacitors, there may not be sufficient
ripple magnitude available on the feedback pin for
stable operation. In this case, an external circuit can be
added to inject ripple voltage into the FB pin.
output ripple voltage, the actual DC output voltage on
V_OUT is offset from the programmed output voltage by the
average value of the output ripple voltage. The initial V_OUT
setting of the regulator can be programmed from 0.6 V to
5.5 V by an external resistor divider (R3 and R4):
4=
3
−1
(13)
where V_REF is 600 mV.
For example; for 1.2 V V_OUT and 10 kΩ R3, then R4 is
10 kΩ. For 600 mV V_OUT, R4 is left open. VFB is
trimmed to a value of 596 mV when V_REF=600 mV, so
the final output voltage, including the effect of the output
ripple voltage, can be approximated by the equation:
=
× 1+
3
4
+
2
(14)
There are some specific considerations when selecting
the RCC ripple injector circuit. For typical applications,
the value of C4 can be selected as 0.1 µF and
approximate values for R2 and C5 can be determined
using the following equations.
Setting the Switching Frequency (f_SW)
f_SW is programmed through external R_FREQ as follows:
= _{20 ×}
×
(15)
R2 must be small enough to develop 12 mV of ripple:
2<
( − )×
× 0.012 × 4 ×
(9)
R2 must be selected such that the R2C4 time constant
enables stable operation:
2 < ^{0.33 × 2}
×f
×
4
×
(10)
The minimum value of C5 can be selected to minimize
the capacitive component of ripple appearing on the
feedback pin:
C5
=L
× C × (R3 + R4)
R2 × R3 × R4 × C4
(11)
Using the minimum value of C5 generally offers the best
transient response, and 100 pF is a good initial value in
many applications. Under some operating conditions,
excessive pulse jitter may be observed. To reduce jitter
and improve stability, the value of C5 can be increased:
5 ≥ 2 × C5
(12)
where C_tON=2.2 pF internal capacitor that generates
t_ON. For example; for f_SW=500 kHz and V_OUT=1.2 V,
select a standard value for R_FREQ=54.9 kΩ.
Inductor Selection
The inductor is typically selected based on the ripple
current (ΔI_L), which is approximately 25% to 45% of the
maximum DC load. The inductor current rating should
be selected such that the saturation and heating current
ratings exceed the intended currents encountered in the
application over the expected temperature range of
operation. Regulators that require fast transient
response use smaller inductance and higher current
ripple; while regulators that require higher efficiency
keep ripple current on the low side.
The inductor value is given by:
=(∆
−
×
)×
(16)
For example: for 12 V V_IN, 1.2 V V_OUT, 15 A load, 25%
ΔI_L, and 500 kHz f_SW; L is 576 nH, and a standard value
of 560 nH is selected.
5 V PV_CC
The PV_CC is supplied from an external source to provide
power to the drivers and V_CC. It is crucial to keep this pin
decoupled to PGND with a ≥1 µF X5R or X7R ceramic
capacitor. Because V_CC powers internal analog circuit, it
is filtered from PV_CC with a 10 Ω resistor and 0.1 µF X7R
decoupling ceramic capacitor to AGND.
Setting the Output Voltage (V_OUT)
The output voltage V_OUT is regulated by initiating a high-
side MOSFET on-time interval when the valley of the
divided output voltage appearing at the FB pin reaches
V_REF. Since this method regulates at the valley of the
Input Capacitor Selection
Input capacitor C_IN is selected based on voltage rating,
RMS current I_CIN(RMS) rating, and capacitance. For
capacitors having DC voltage bias derating, such as
ceramic capacitors, higher rating is strongly
recommended. RMS current rating is given by:
( )⁼
× × (1 − )
(17)
where I_LOAD-MAX is the maximum load current and D is
the duty cycle V_OUT/V_IN. The maximum I_CIN(RMS) occurs
at 50% duty cycle.
© 2011 Fairchild Semiconductor Corporation
FAN2315 • Rev. 1.0.4
13
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