Applications Information: continued
C
OFF timing capacitor:
TJUNCTION(MAX) - TAMBIENT
Thermal Impedance =
Power
Period × (1 - duty cycle)
COFF
=
,
4848.5
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.
where:
1
Period =
switching frequency
EMI Management
“Droop” Resistor for Adaptive Voltage Positioning
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.
Adaptive voltage positioning is used to reduce output volt-
age excursions during abrupt changes in load current.
Regulator output voltage is offset +40mV when the regula-
tor is unloaded, and -40mV at full load. This results in
increased margin before encountering minimum and maxi-
mum transient voltage limits, allowing use of less capaci-
tance on the regulator output (see Figure 7).
To implement adaptive voltage positioning, a “droop”
resistor must be connected between the output inductor
and output capacitors and load. This is normally imple-
mented by a PC board trace of the following value:
80mV
IMAX
RDROOP
=
Adaptive voltage positioning can be disabled for improved
DC regulation by connecting the VFB pin directly to the load
using a separate, non-load current carrying circuit trace.
2µH
2µH
Input and Output Capacitors
+
1200µF x 3/16V
33Ω
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.
1000pF
Figure 16: Filter components
Figure 17: Input Filter
Layout Guidelines
1. Place 12V filter capacitor next to the IC and connect
Output Inductor
capacitor ground to pin 11 (PGnd).
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.
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 (VFFB
)
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.
Thermal Management
Thermal Considerations for Power MOSFETs and Diodes
5. Place the output filter capacitor(s) as close to the load as
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:
possible and connect the ground terminal to pin 14 (LGnd).
6. To implement adaptive voltage positioning, connect
both slow and fast feedback pins 16 (VFB) and 8 (VFFB) to
the regulator output right at the inductor terminal. Connect
inductor to the output capacitors via a trace with the fol-
lowing resistance:
11
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
