RT8280 RICHTEK [Richtek Technology Corporation], RT8280 Datasheet - Page 12

• Manufacturer Part Number: RT8280
• Description: 3A, 24V, 3MHz Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

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, V
equal to ΔI
or discharged generating a feedback error signal for the
regulator to return V
this recovery time, V
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 :
where
g
g
Once the value of Rc has been determined, the value of
Cc can be obtained by the following equation :
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12
R
C
CS
EA
C
C
=
is Current SenseTransconductance = 1.8 (A/V)
is Error Amplifier Tansconductance = 920 (μA/V)
=
2
2
©
π
π
4.5V to 24V
2012 Richtek Technology Corporation. All rights reserved.
Chip Enable
×
g
×
LOAD
CS
C
R
1
OUT
C
×
×
(ESR) and C
g
V
f
EA
IN
4
C
×
f
* : Optional
C
×
OUT
OUT
OUT
×
V
FB
V
R
OUT
C
immediately shifts by an amount
to its steady state value. During
can be monitored for overshoot
EN
Figure 6. Reference Circuit with Snubber and Enable Timing Control
EN
*
OUT
*
R
24k
RT
also begins to be charged
9 (Exposed Pad)
10µF
C
IN
4,
2
7
8
VIN
EN
RT
GND
RT8280
COMP
BOOT
SW
Choose a capacitor that is greater than the above
calculation result. The frequency of the zero, which
consists of R
f
to f
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. C
can be used to produce a pole with R
To calculate C
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, C
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.
FB
C
C
P
to get a sufficient phase margin. If the zero moves close
C
1
3
5
6
=
, the phase margin decreases.
is a registered trademark of Richtek Technology Corporation.
C
R
OUT
BOOT
R
C
S
S
R
*
*
BOOT
×
C
*
1.8nF
C
ESR
C
P
and C
C
B330A
, follow the equation below :
C
10nF
. But this method will decrease the driving
BOOT
C
P
24k
R
C
C
2.2µH
, should be lower than one fourth of
L
NC
10k
R1
31.6k
R2
DS8280-02 March 2012
C
C
22µF
to cancel the zero.
OUT
V
3.3V/3A
OUT
P