ISL85402 データシートの表示（PDF） - Renesas Electronics
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ISL85402
Renesas Electronics
ISL85402 Datasheet PDF : 22 Pages
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ISL85402
reflect reality. As a result, the actual capacitance may be
considerably lower than the advertised value. Consult the
manufacturers data sheet to determine the actual in-application
capacitance. Most manufacturers publish capacitance vs DC bias
so that this effect can be easily accommodated. The effects of
AC voltage are not frequently published, but an assumption of
~20% further reduction will generally suffice. The result of these
considerations can easily result in an effective capacitance 50%
lower than the rated value. Nonetheless, they are a very good
choice in many applications due to their reliability and extremely
low ESR.
The following equations allow calculation of the required
capacitance to meet a desired ripple voltage level. Additional
capacitance may be used.
For the ceramic capacitors (low ESR):
VOUTripple=
-------------------I---------------
8
FS
W
COU
T
(EQ. 11)
where I is the inductor’s peak to peak ripple current, FSW is the
switching frequency and COUT is the output capacitor.
If using electrolytic capacitors then:
VOUTripple= I*ESR
(EQ. 12)
Regarding transient response needs, a good starting point is to
determine the allowable overshoot in VOUT if the load is suddenly
removed. In this case, energy stored in the inductor will be
transferred to COUT causing its voltage to rise. After calculating
capacitance required for both ripple and transient needs, choose
the larger of the calculated values. The following equation
determines the required output capacitor value in order to
achieve a desired overshoot relative to the regulated voltage.
COUT
=
--------------------------------I-O----U----T---2----*----L--------------------------------
VOUT2*VOUTMAX VOUT2 – 1
(EQ. 13)
where VOUTMAX/VOUT is the relative maximum overshoot
allowed during the removal of the load.
Input Capacitors
Depending on the system input power rail conditions, the
aluminum electrolytic type capacitor is normally needed to
provide the stable input voltage. Thus, restrict the switching
frequency pulse current in a small area over the input traces for
better EMC performance. The input capacitor should be able to
handle the RMS current from the switching power devices.
Ceramic capacitors must be used at VIN pin of the IC and
multiple capacitors including 1µF and 0.1µF are recommended.
Place these capacitors as closely as possible to the IC.
Buck Output Inductor
The inductor value determines the converter’s ripple current.
Choosing an inductor current requires a somewhat arbitrary
choice of ripple current, I. A reasonable starting point is 30% to
40% of total load current. The inductor value can then be
calculated using Equation 14:
L
=
-V---I--N-----–----V----O---U----T-
Fs I
-V---O----U---T-
VIN
(EQ. 14)
Increasing the value of inductance reduces the ripple current and
thus ripple voltage. However, the larger inductance value may
reduce the converter’s response time to a load transient. The
inductor current rating should be such that it will not saturate in
overcurrent conditions.
Low-Side Power MOSFET
In synchronous buck application, a power N MOSFET is needed
as the synchronous low side MOSFET and a good one should
have low Qgd, low rDS(ON) and small Rg (Rg_typ < 1.5Ω
recommended). Vgth_min is recommended to be higher than
1.2V. A good example is SQS462EN.
Output Voltage Feedback Resistor Divider
The output voltage can be programmed down to 0.8V by a
resistor divider from VOUT to FB according to Equation 15.
VOUT
=
0.8
1
+
----R-----U-----P------
RLOW
(EQ. 15)
In an application requiring least input quiescent current, large
resistors should be used for the divider. 232k is recommended
for the upper resistor.
Loop Compensation Design
The ISL85402 uses constant frequency peak current mode
control architecture to achieve fast loop transient response. An
accurate current sensing pilot device in parallel with the upper
MOSFET is used for peak current control signal and overcurrent
protection. The inductor is not considered as a state variable
since its peak current is constant, and the system becomes
single order system. It is much easier to design the compensator
to stabilize the loop compared with voltage mode control. Peak
current mode control has inherent input voltage feed-forward
function to achieve good line regulation. Figure 29 shows the
small signal model of a buck regulator.
^iin
V^in
+
^iL LP
RLP
ILd^ 1:D Vind^
RT
vo^
Rc
Ro
Co
d^
Fm
T i(S)
+
He(S)
Tv(S)
v^comp -Av(S)
FIGURE 29. SMALL SIGNAL MODEL OF BUCK REGULATOR
FN7640 Rev 1.00
April 25, 2013
Page 17 of 22
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