RT8252B Datasheet, PDF (11/14 Pages) Richtek Technology Corporation – 2A, 23V, 1.2MHz Synchronous Step-Down Converter

RT8252B
response as described in a later section.
The output ripple, ΔV_OUT , is determined by :
ΔV_OUT
≤
ΔI_L
⎡⎢⎣_{ESR +}
1
8fC_OUT
⎤
⎥⎦
input, V_IN. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at V_IN large enough to damage the
part.
The output ripple will be highest at the maximum input
voltage since ΔI_L increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Dry tantalum,
special polymer, aluminum electrolytic and ceramic
capacitors are all available in surface mount
packages.Special polymer capacitors offer very low ESR
value. However, it provides lower capacitance density than
other types. Although Tantalum capacitors have the highest
capacitance density, it is important to only use types that
pass the surge test for use in switching power supplies.
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_OUT immediately shifts by an amount
equal to ΔI_LOAD (ESR) also begins to charge or discharge
C_OUT generating a feedback error signal for the regulator
to return V_OUT to its steady-state value. During this
recovery time, V_OUT can be monitored for overshoot or
ringing that would indicate a stability problem.
Aluminum electrolytic capacitors have significantly higher
ESR. However, it can be used in cost-sensitive applications
for ripple current rating and long term reliability
considerations. Ceramic capacitors have excellent low
ESR characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
EMI Consideration
Since parasitic inductance and capacitance effects in PCB
circuitry would cause a spike voltage on SW pin when
high side MOSFET is turned-on/off, this spike voltage on
SW may impact on 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 make them as close
as possible to the SW pin (see Figure 5). Another method
is adding a resistor in series with the bootstrap
capacitor, C_BOOT. 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 on EMI performance. For detailed PCB layout
guide, please refer to the section of Layout Consideration.
V_IN
4.5V to 23V
Chip Enable
R_EN*
C_IN
10µF
2 _VIN
BOOT ^{1 RBOOT*}
RT8252B
7 _EN
SW ³
C_{BOOT L}
100nF ^3.6µH
C_EN*
R_S*
R1
C_S*
75k
8 _SS
C_SS
4,
0.1µF _{Exposed Pad(9)}
GND
FB ⁵
COMP ⁶
C_C
0.82nF
R_C
32k
R2
24k
V_OUT
3.3V/2A
C_OUT
22µFx2
* : Optional
C_P
NC
Figure 5. Reference Circuit with Snubber and Enable Timing Control
DS8252B-02 March 2011
www.richtek.com
11