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ISL78206AVEZ Datasheet(PDF) 15 Page - Intersil Corporation

Part No. ISL78206AVEZ
Description  40V 2.5A Buck Controller with Integrated High-side 40V 2.5A Buck Controller with Integrated High-side
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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ISL78206AVEZ Datasheet(HTML) 15 Page - Intersil Corporation

 
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ISL78206
15
FN8618.2
March 25, 2015
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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.
Where VOUTMAX/VOUT is the relative maximum overshoot
allowed during the removal of the load.
Input Capacitors
Depending upon the system input power rail conditions, the
aluminum electrolytic type capacitor is normally needed to
provide the stable input voltage and restrict the switching
frequency pulse current in small areas 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 the 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.
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 7:
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 or higher than
1.2V. A good example is SQS462EN.
In synchronous buck configuration, a 5.1k or smaller value
resistor has to be added to connect LGATE to ground to avoid
falsely turn-on of LGATE caused by coupling noise.
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 8.
In applications requiring the least input quiescent current, large
resistors should be used for the divider to keep its leakage
current low. Generally, a resistor value of 10k to 300k can be
used for the upper resistor.
Loop Compensation Design
The ISL78206 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 22 shows the
small signal model of a buck regulator.
PWM Comparator Gain Fm
The PWM comparator gain Fm for peak current mode control is
given by Equation 9:
Where Se is the slew rate of the slope compensation and Sn is
given by Equation 10.
Where Rt is the gain of the current amplifier.
Current Sampling Transfer Function He(S)
In current loop, the current signal is sampled every switching
cycle. It has the following transfer function in Equation 11:
Where Qn and n are given by
(EQ. 6)
COUT
IOUT2*L
VOUT2* VOUTMAX VOUT
2 1 
--------------------------------------------------------------------------------------------
=
(EQ. 7)
L
VIN VOUT
Fs
I
--------------------------------
VOUT
VIN
----------------
=
VOUT
0.8
1
RUP
RLOW
-----------------
+



=
(EQ. 8)
d
Vin
d
IL
in
in
i
L
+
1:D
L
i
Co
Rc
-Av(S)
d
comp
v
R T
Fm
He(S)
+
T i(S)
o
v
Tv(S)
I
LP
+
1:D
Rc
Ro
-Av(S)
R T
Fm
He(S)
T i(S)
o
T(S)
^
^
V
^
^
^
^
^
^
FIGURE 22. SMALL SIGNAL MODEL OF BUCK REGULATOR
RLP
Fm
comp
-----------------
1
Se Sn
+
T
s
--------------------------------
==
(EQ. 9)
Sn
Rt
Vin Vo
LP
---------------------
=
(EQ. 10)
He S
 S
2
n
2
-------
=
S
nQn
--------------- 1
++
(EQ. 11)
Qn
2
---
=
n
f
s
=


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