CS5150
Applications Information: continued
Input and Output Capacitors
These components must be selected and placed carefully to
yield optimal results. Capacitors should be chosen to pro-
vide acceptable ripple on the input supply lines and regula-
tor output voltage. Key specifications for input capacitors
are their ripple rating, while ESR is important for output
capacitors. For best transient response, a combination of
low value/high frequency and bulk capacitors placed close
to the load will be required.
Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the induc-
tor value will decrease output voltage ripple, but degrade
transient response.
Thermal Management
Thermal Considerations for Power MOSFETs and Diodes
In order to maintain good reliability, the junction tempera-
ture of the semiconductor components should be kept to a
maximum of 150°C or lower. The thermal impedance (junc-
tion to ambient) required to meet this requirement can be
calculated as follows:
T
JUNCTION(MAX)
- T
AMBIENT
Thermal Impedance =
Power
A heatsink may be added to TO-220 components to reduce
their thermal impedance. A number of PC board layout
techniques such as thermal vias and additional copper foil
area can be used to improve the power handling capability
of surface mount components.
EMI Management
As a consequence of large currents being turned on and off
at high frequency, switching regulators generate noise as a
consequence of their normal operation. When designing for
compliance with EMI/EMC regulations, additional com-
ponents may be added to reduce noise emissions. These
components are not required for regulator operation and
experimental results may allow them to be eliminated. The
input filter inductor may not be required because bulk filter
and bypass capacitors, as well as other loads located on the
board will tend to reduce regulator di/dt effects on the cir-
cuit board and input power supply. Placement of the
power component to minimize routing distance will also
help to reduce emissions.
Layout Guidelines
1. Place 12V filter capacitor next to the IC and connect
capacitor ground to pin 11 (PGnd).
2. Connect pin 11 (PGnd) with a separate trace to the
ground terminals of the 5V input capacitors.
3. Place fast feedback filter capacitor next to pin 8 (V
FFB
)
and connect its ground terminal with a separate, wide trace
directly to pin 14 (LGnd).
4. Connect the ground terminals of the Compensation
capacitor directly to the ground of the fast feedback filter
capacitor to prevent common mode noise from effecting
the PWM comparator.
5. Place the output filter capacitor(s) as close to the load as
possible and connect the ground terminal to pin 14 (LGnd).
6. To implement adaptive voltage positioning, connect
both slow and fast feedback pins 16 (V
FB
) and 8 (V
FFB
) to
the regulator output right at the inductor terminal. Connect
inductor to the output capacitors via a trace with the fol-
lowing resistance:
R
TRACE
=
80mV
I
MAX
This causes the output voltage to be +40mV with no load,
and -40mV with a full load, improving regulator transient
response. This trace must be wide enough to carry the full
output current. (Typical trace is 1.0 inch long, 0.17 inch
wide). Care should be taken to minimize any additional
losses after the feedback connection point to maximize reg-
ulation.
7. If DC regulation is to be optimized (at the expense of
degraded transient regulation), adaptive voltage position-
ing can be disabled by connecting to V
FB
pin directly to the
load with a separate trace (remote sense).
8. Place 5V input capacitors close to the switching MOSFET
and synchronous MOSFET.
Route gate drive signals V
GATE(H)
(pin 10) and V
GATE(L)
(pin 12 when used) with traces that are a minimum of 0.025
inches wide.
V
CC
0.1µF
15
To the negative terminal of the
input capacitors
11
1.0µF
V
COMP
100pF
V
FFB
8
5
SOFTSTART
OFF TIME
To the negative terminal of the output capacitors
2µH
2µH
+
Figure 20: Layout Guidelines
33Ω
1000pF
1200µF x 3/16V
Figure 18: Filter components
Figure 19: Input Filter
12
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
