RT8280 데이터 시트보기 (PDF) - Richtek Technology
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RT8280 Datasheet PDF : 16 Pages
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RT8280
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD (ESR) and COUT also begins to be charged
or discharged generating a feedback error signal for the
regulator to return VOUT to its steady state value. During
this recovery time, VOUT can be monitored for overshoot
or ringing to indicate any stability problem.
Compensation Parameters
The switching frequency of the RT8280 can be programmed
from free running frequency to 3MHz. Table 1 only lists
the recommended compensation parameters for 2.2MHz
switching frequency. Optimized compensation parameters
for other switching frequency can also be determined
through below procedures. The first step is to decide the
crossover frequency, fc. In general, the crossover
frequency is one tenth of the switching frequency. Then,
Rc can be obtained through the following equation :
RC
=
2π
× COUT × fC × VOUT
gCS × gEA × VFB
where
gCS is Current SenseTransconductance = 1.8 (A/V)
gEA is Error Amplifier Tansconductance = 920 (μA/V)
Once the value of Rc has been determined, the value of
Cc can be obtained by the following equation :
CC
=
2π
1
× RC
×
fC
4
Choose a capacitor that is greater than the above
calculation result. The frequency of the zero, which
consists of RC and CC, should be lower than one fourth of
fC to get a sufficient phase margin. If the zero moves close
to fC, the phase margin decreases.
In some applications, the output capacitor will be an
electrolytic capacitor, not a ceramic capacitor. A zero will
be produced by the electrolytic capacitor and its ESR. CP
can be used to produce a pole with RC to cancel the zero.
To calculate CP, follow the equation below :
CP
=
COUT ×ESR
RC
EMI Consideration
Since parasitic inductance and capacitance effects in PCB
circuitry would cause a spike voltage on the SW pin when
the high side MOSFET is turned-on/off, this spike voltage
on SW may impact EMI performance in the system. In
order to enhance EMI performance, there are two methods
to suppress the spike voltage. One is to place an R-C
snubber between SW and GND and place them as close
as possible to the SW pin (see Figure 6). Another method
is to add a resistor in series with the bootstrap
capacitor, CBOOT. But this method will decrease the driving
capability to the high side MOSFET. It is strongly
recommended to reserve the R-C snubber during PCB
layout for EMI improvement. Moreover, reducing the SW
trace area and keeping the main power in a small loop will
be helpful for EMI performance. For detailed PCB layout
guide, please refer to the section on Layout Consideration.
VIN
4.5V to 24V
Chip Enable
REN*
CEN*
RRT
24k
CIN
10µF
2 VIN
BOOT 1
7 EN
RT8280
SW 3
8 RT
4,
9 (Exposed Pad)
GND
FB 5
COMP 6
RBOOT*
CBOOT L
10nF 2.2µH
RS* B330A
CS*
R1
31.6k
CC
1.8nF
RC
24k
R2
10k
NC
VOUT
3.3V/3A
COUT
22µF
* : Optional
CP
Figure 6. Reference Circuit with Snubber and Enable Timing Control
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
12
is a registered trademark of Richtek Technology Corporation.
DS8280-02 March 2012
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