ISL78228 Datasheet, PDF (16/18 Pages) Intersil Corporation – Dual 800mA Low Quiescent Current 2.25MHz High Efficiency Synchronous Buck Regulator

ISL78228
ISL78228 uses internal compensation network and the output
capacitor value is dependent on the output voltage. The ceramic
capacitor is recommended to be X5R or X7R. The recommended
minimum output capacitor values are shown in Table 2 for the
ISL78228.
TABLE 2. OUTPUT CAPACITOR VALUE vs V_OUT ISL78228
V_OUT
(V)
C_OUT
L
(µF)
(µH)
0.8
10
1.0~2.2
1.2
10
1.0~2.2
1.6
10
1.0~2.2
1.8
10
1.5~3.3
2.5
10
1.5~3.3
3.3
6.8
1.5~4.7
3.6
8.6
1.5~4.7
In Table 2, the minimum output capacitor value is given for
different output voltages to make sure the whole converter
system is stable.
Output Voltage Selection
The output voltage of the regulator can be programmed via an
external resistor divider that is used to scale the output voltage
relative to the internal reference voltage and feed it back to the
inverting input of the error amplifier. Refer to “Typical
Application” on page 2.
The output voltage programming resistor, R₂ (or R₅ in
Channel 2), will depend on the desired output voltage of the
regulator. The value for the feedback resistor is typically between
0Ω and 750kΩ, as shown in Equation 2.
Let R₃ = 100kΩ, then R₂ will be:
R₂
=
R₃
⎛
⎜
⎝
V_{----O----U---T-}
V_FB
–
⎞
1⎟
⎠
(EQ. 2)
If the output voltage desired is 0.6V, then R₃ is left unpopulated
and short R₂. For faster response performance, add 47pF in
parallel to R₂.
Input Capacitor Selection
The main functions of the input capacitor are to provide
decoupling of the parasitic inductance and to provide filtering
function to prevent the switching current flowing back to the
battery rail. One 10µF X5R or X7R ceramic capacitor is a good
starting point for the input capacitor selection for both channels.
PCB Layout Recommendation
The PCB layout is a very important converter design step to make
sure the designed converter works well. For ISL78228, the power
loop is composed of the output inductor (L’s), the output
capacitor (C_OUT1 and C_OUT2), the LX’s pins, and the GND pin. It is
necessary to make the power loop as small as possible and the
connecting traces among them should be direct, short and wide.
The switching node of the converter, the LX_ pins, and the traces
connected to the node are very noisy, so keep the voltage
feedback trace away from these noisy traces. The input capacitor
should be placed as closely as possible to the VIN pin. The ground
of input and output capacitors should be connected as closely as
possible. The heat of the IC is mainly dissipated through the
thermal pad. Maximizing the copper area connected to the
thermal pad is preferable. In addition, a solid ground plane is
helpful for better EMI performance. It is recommended to add at
least 5 vias ground connection within the pad for the best
thermal relief.
16
FN7849.0
May 2, 2011