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NCP3011, NCV3011
High ? Side
Logic Signal
Low ? Side
Logic Signal
R DSmax
High ? Side
MOSFET
t d(on)
NOL HL
t f
NOL LH
R DS(on)min
R DSmax
Low ? Side
MOSFET
t r
t d(off)
t r
t f
R DS(on)min
t d(on)
t d(off)
Figure 44. MOSFETs Timing Diagram
f P0 +
f Z0 +
2 @ p @ C OUT @ ESR
Another consideration during MOSFET selection is their
delay times. Turn ? on and turn ? off times must be short
enough to prevent cross conduction. If not, there will be
conduction from the input through both MOSFETs to
ground. Therefore, the following conditions must be met.
t d(ON)_CONTROL ) NOL LH u t d(OFF)_SYNC ) t f _SYNC
and (eq. 40)
t (ON)_SYNC ) NOL HL u t d(OFF)_CONTROL ) t f _CONTROL
The MOSFET parameters, t d(ON) , t r , t d(OFF) and t f are can
be found in their appropriate datasheets for specific
conditions. NOL LH and NOL HL are the dead times which
were described earlier and are 85 ns and 75 ns, respectively.
Feedback and Compensation
The NCP3011 is a voltage mode buck convertor with a
transconductance error amplifier compensated by an
external compensation network. Compensation is needed to
achieve accurate output voltage regulation and fast transient
Table 2. COMPENSATION TYPES
response. The goal of the compensation circuit is to provide
a loop gain function with the highest crossing frequency and
adequate phase margin (minimally 45 ° ). The transfer
function of the power stage (the output LC filter) is a double
pole system. The resonance frequency of this filter is
expressed as follows:
1
(eq. 41)
2 @ p @ L @ C OUT
Parasitic Equivalent Series Resistance (ESR) of the
output filter capacitor introduces a high frequency zero to
the filter network. Its value can be calculated by using the
following equation:
1
(eq. 42)
The main loop zero crossover frequency f 0 can be chosen
to be 1/10 ? 1/5 of the switching frequency. Table 2 shows
the three methods of compensation.
Zero Crossover Frequency Condition
f P0 < f Z0 < f 0 < f S /2
f P0 < f 0 < f Z0 < f S /2
f P0 < f 0 < f S /2 < f Z0
Compensation Type
Type II
Type III Method I
Type III Method II
http://onsemi.com
25
Typical Output Capacitor Type
Electrolytic, Tantalum
Tantalum, Ceramic
Ceramic
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