ISL78201 Datasheet, PDF (19/23 Pages) Intersil Corporation – 40V 2.5A Regulator with Integrated High-side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78201
For the ceramic capacitors (low ESR):
V_OUTripple=
---------------^----^I--------------
8^ f _{S W}^ C_{O U T}
(EQ. 15)
Where I is the inductor’s peak to peak ripple current, f_SW is the
switching frequency and C_OUT is the output capacitor.
If using electrolytic capacitors then:
V_OUTripple= I*ESR
(EQ. 16)
Regarding transient response needs, a good starting point is to
determine the allowable overshoot in V_OUT if the load is suddenly
removed. In this case, energy stored in the inductor will be
transferred to C_OUT causing its voltage to rise. After calculating
capacitance required for both ripple and transient needs, choose
the larger of the calculated values. The Equation 17 determines
the required output capacitor value in order to achieve a desired
overshoot relative to the regulated voltage.
C_OUT
=
--------------------------------^I-^O----^U----^T---²----^*----^L--------------------------------
V_OUT^2*V_OUTMAX  V_OUT² – 1 
(EQ. 17)
where V_OUTMAX/V_OUT is the relative maximum overshoot
allowed during the removal of the load.
Input Capacitors - Buck
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 - Buck
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 18:
L
=
-^V---^I--^N-----–----^V----^O---^U----^T-
Fs  I

-^V---^O----^U---^T-
V_IN
(EQ. 18)
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 r_DS(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.
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 V_OUT to FB according to Equation 19.
V_OUT
=
0.8




1
+
R^---R--_L^--U-_O^--P-_W^--- _{}
(EQ. 19)
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.
Boost Inductor
Besides the need to sustain the current ripple to be within a
certain range (30% to 50%), the boost inductor current at its
soft-start is a more important perspective to be considered in
selection of the boost inductor. Each time the boost starts up,
there is a fixed 500µs soft-start time when the duty cycle
increase linearly from t_MINON to ~50%. Before and after boost
start-up, the boost output voltage will jump from V_IN_boost to
voltage (V_IN_boost + V_OUT_buck). The design target in boost
soft-start is to ensure the boost input current is sustained to a
minimum but capable of charging the boost output voltage to
have a voltage step equaling to V_OUT_buck. A big inductor will
block the inductor current increase and not high enough to be
able to charge the output capacitor to the final steady state value
(V_IN_boost+V_OUT_buck) within 500µs. A 6.8µH inductor is a good
starting point for its selection in design. The boost inductor
current at start-up must be checked by an oscilloscope to ensure
it is under the acceptable range. It is suggested to run the iSim
model simulation to select the proper inductor value.
Boost Output Capacitor
Based on the same theory in boost start-up described above in
boost inductor selection, a large capacitor at boost output will
cause high inrush current at boost PWM start-up. 22µF is a good
choice for applications with buck output voltage less than 10V.
Also, some minimum amount of capacitance has to be used in
boost output to keep the system stable. It is suggested to run the
iSim model simulation to select the proper inductor value.
Loop Compensation Design-Buck
The ISL78201 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 35 shows the
small signal model of a buck regulator.
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FN8615.1
March 31, 2015