NCP1573D ONSEMI [ON Semiconductor], NCP1573D Datasheet - Page 13

• Manufacturer Part Number: NCP1573D
• Description: Low Voltage Synchronous Buck Controller
• Manufacturer: ONSEMI [ON Semiconductor]

current rating for a 6.3 V, 1800 nF capacitor is 2000 mA at
100 kHz and 105°C. We determine the number of input
capacitors by dividing the ripple current by the
per−capacitor current rating:
meet the input capacitor ripple current requirements.
Output Switch FETs
properties vary widely from manufacturer to manufacturer.
The NCP1573 system is designed assuming that n−channel
FETs will be used. The FET characteristics of most concern
are the gate charge/gate−source threshold voltage, gate
capacitance, on−resistance, current rating and the thermal
capability of the package.
the switch FET has a high gate charge, the amount of time
the FET stays in its ohmic region during the turn−on and
turn−off transitions is larger than that of a low gate charge
FET, with the result that the high gate charge FET will
consume more power. Similarly, a low on−resistance FET
will dissipate less power than will a higher on−resistance
FET at a given current. Thus, low gate charge and low
R
generated heat.
reduce power consumption. By placing a number of FETs in
parallel, the effective R
ohmic power loss. However, placing FETs in parallel
increases the gate capacitance so that switching losses
increase. As long as adding another parallel FET reduces the
ohmic power loss more than the switching losses increase,
there is some advantage to doing so. However, at some point
the law of diminishing returns will take hold, and a marginal
increase in efficiency may not be worth the board area
required to add the extra FET. Additionally, as more FETs
are used, the limited drive capability of the FET driver will
have to charge a larger gate capacitance, resulting in
increased gate voltage rise and fall times. This will affect the
amount of time the FET operates in its ohmic region and will
increase power dissipation.
dissipation in the switch FETs.
I IN(RMS) +
DS(ON)
Input capacitor RMS ripple current is then
If we consider a Rubycon MBZ series capacitor, the ripple
A total of at least 3 capacitors in parallel must be used to
Output switch FETs must be chosen carefully, since their
The onboard FET driver has a limited drive capability. If
It can be advantageous to use multiple switch FETs to
The following equations can be used to calculate power
For ohmic power losses due to R
P ON(TOP) +
Number of capacitors + 4.74 A 2.0 A + 2.3
will result in higher efficiency and will reduce
I IN(AVE) + (10 A)(3.3 V 5 V) + 6.6 A
+ 4.74 A
6.6 2 ) 3.3 V
[( 10 A * 6.6 A ) 2 * 6.6 A 2 ]
( R DS(ON)(TOP) )( I RMS(TOP) ) 2
DS(ON)
( number of topside FETs )
5 V
is reduced, thus reducing the
DS(ON)
:
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13
where:
practice to use the value of R
junction temperature in the calculations shown above.
P ON(BOTTOM) +
I RMS(BOTTOM) + I
n = number of phases.
Note that R
I RMS(TOP) +
DS(ON)
R DS(ON)(BOTTOM) I RMS(BOTTOM) 2
increases with temperature. It is good
2
PK
I
2
PK
* (I PK I RIPPLE ) )
number of bottom−side FETs
* (I PK )(I RIPPLE ) ) D
DS(ON)
at the FET’s maximum
(1 * D)
3
3
I
2
RIPPLE
I
2
RIPPLE