ISL78200_14 Datasheet, PDF (17/22 Pages) Intersil Corporation – 2.5A Regulator with Integrated High-Side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78200
Output Capacitors
An output capacitor is required to filter the inductor current.
Output ripple voltage and transient response are 2 critical factors
when considering output capacitance choice. The current mode
control loop allows the usage of low ESR ceramic capacitors and
thus smaller board layout. Electrolytic and polymer capacitors
may also be used.
Additional consideration applies to ceramic capacitors. While
they offer excellent overall performance and reliability, the actual
in-circuit capacitance must be considered. Ceramic capacitors
are rated using large peak-to-peak voltage swings and with no DC
bias. In the DC/DC converter application, these conditions do not
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):
V_OUTripple=
---------------^----^I---------------
8^
F_S

W
C_OU
T
(EQ. 11)
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. 12)
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 following equation
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. 13)
where V_OUTMAX/V_OUT 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:
L
=
V_{----I--N-----–----}V_{----O---U----T-}
Fs  I

-^V---^O----^U---^T-
V_IN
(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 r_DS(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 V_OUT to FB according to Equation 15.
V_OUT
=
0.8




1
+
R^---R--_L^--U-_O^--P-_W^--- _{}
(EQ. 15)
In applications requiring the least input quiescent current, large
resistors should be used for the divider to keep its leakage
current low. 232k is a recommended for the upper resistor.
Loop Compensation Design
The ISL78200 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 32 shows the
small signal model of a buck regulator.
17
FN7641.2
December 24, 2013