SC471AEVB datasheet(20/27 Pages) SEMTECH

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SC471/SC471A

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POWER MANAGEMENT

will change the output ripple and thus the DC output

voltage. The output ESR also affects the ripple and thus

the DC output voltage.

Switching Frequency Variations

The switching frequency will vary somewhat due to line

and load conditions. The line variations are a result

of a ﬁxed offset in the on-time one-shot, as well as

unavoidable delays in the external MOSFET switching. As

VBAT increases, these factors make the actual DH on-time

slightly longer than the idealized on-time. The net effect

is that frequency tends to fall slightly with increasing input

voltage.

The load variations are due to losses in the power train

from IR drop and switching losses. For a conventional

PWM constant-frequency topology, as load increases

the duty cycle also increases slightly to compensate for

IR and switching losses in the MOSFETs and inductor. A

constant on-time topology must also overcome the same

losses by increasing the duty cycle (more time is spent

drawing energy from VBAT as losses increase). Since the

on-time is constant for a given VOUT/VBAT combination,

the way to increase duty cycle is to gradually shorten

the off-time. The net effect is that switching frequency

increases slightly with increasing load.

Layout Guidelines

One or more ground planes are recommended to minimize

the effect of switching noise and copper losses and to

maximize heat removal. The analog ground reference,

RTN, should connect directly to the thermal pad, which

in turn connects to the ground plane through preferably

one large via. There should be a RTN plane or copper

are near the chip; all components that are referenced to

RTN should connect to this plane directly, not through the

ground plane, and located on the chip side of the PCB if

possible.

GND should be a separate plane which is not used for

routing analog traces. The VCC input provides power to

the internal analog circuits and the upper and lower gate

drivers.

The VCC supply decoupling capacitor should be tied

between VCC and GND with short traces. All power GND

connections should connect directly to this plane with

special attention given to avoiding indirect connections

between RTN and GND which will create ground loops.

As mentioned above, the RTN plane must be connected to

the GND plane at the chip near the RTN/GND pins.

The switcher power section should connect directly to the

ground plane(s) using multiple vias as required for current

handling (including the chip power ground connections).

Power components should be placed to minimize loops

and reduce losses. Make all the power connections on

one side of the PCB using wide copper ﬁlled areas if

possible. Do not use “minimum” land patterns for power

components. Minimize trace lengths and maximize trace

widths between the gate drivers and the gates of the

MOSFETs to reduce parasitic impedances (and MOSFET

switching losses); the low-side MOSFET is most critical.

Maintain a length to width ratio of <20:1 for gate drive

signals. Use multiple vias as required by current handling

requirement (and to reduce parasitic) if routed on more

than one layer.

For an accurate ILIM current sense connection, connect

the ILIM trace to the current sense element (MOSFET or

resistor) directly at the pin of the element, and route that

trace over to the ILIM resistor on another layer if needed.

The layout can be generally considered in two parts; the

analog control section referenced to RTN, and the switcher

power section referenced to GND.

Looking at the control section ﬁrst, locate all components

referenced to RTN on the schematic and place these

components near the chip and on the same side if

possible. Connect RTN using a wide trace. Very little

current ﬂows in the RTN path and therefore large areas

of copper are not needed. Connect the RTN pin directly to

the thermal pad under the device as the only connection

between RTN and GND.

The chip supply decoupling capacitor (VCC/GND) should

be located near to the pins. Since the DL pin is directly

between VCC and GND, and the DL trace must be a wide,

direct trace, the VCC decoupling capacitor is best placed

on the opposite side of the PCB, routed with traces as short

as possible and using at least two vias when connecting

through the PCB.

Applications Information (continued)

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