SC418 Datasheet, PDF (21/30 Pages) Semtech Corporation – EcoSpeedTM DC-DC Buck Controller with Programmable LDO

SC418
Applications Information (continued)
It is not recommended to use the switch-over feature for
an output voltage of 3.3V or less since this does not
provide sufficient voltage for the gate-source drive to the
internal p-channel switch-over MOSFET.
It is not recommended to use the switch-over feature in
cases where the device is powered from 3.3V which is ini-
tially derived from the LDO. If the switch-over was used,
then the 3.3V switcher output would pass through the
switch-over MOSFET to power the VDDA/VDDP pins. The
RDS_(ON) of the switch-over device, typically 2.2 ohms at
3.3V, will cause a voltage drop across the device. The
VDDA/VDDP pins would be typically 200mV or more
below the 3.3V rail, due to the IR drop caused by the
VDDA/VDDP current. If this voltage drop becomes large
enough, the VDDA voltage will drop near the VDDA UVLO
threshold, causing the device to shutdown.
Switch-over MOSFET Parasitic Diodes
The switch-over MOSFET contains parasitic diodes that
are inherent to its construction, as shown in Figure 13.
Switchover
control
V_LDO
Switchover
MOSFET
V_OUT
will forward-bias one of the parasitic diodes and could
damage the device.
Using the Internal LDO to Bias the SC418
The following steps must be followed when using the
internal LDO to bias the device.
• Connect VDDA and VDDP to VLDO before
enabling the LDO.
• Any external load on VLDO should not exceed
40mA until the LDO voltage has reached 90% of
final value.
When the switch-over feature is used and the VDDA/VDDP
power comes from V_OUT, the EN and ENL inputs must be
used carefully. Do not connect the EN pin directly to VDDA
or another supply voltage. If this is done, driving the ENL
pin low (to AGND) will turn off the LDO and the LDO
switch-over MOSFET, but the switcher will continue oper-
ating. V_OUT will feed into the LDO output and the VDDA/
VDDP supplies through the internal parasitic diode. This
can potentially damage the device, and also prevents the
switcher from shutting off until the VDDA supply drops
below the VDDA UVLO threshold. For these applications a
dedicated logic signal is required to drive EN low and
disable the switcher. This signal can be combined with
the ENL signal if needed.
Parasitic diode
Parasitic diode
VDDA
Figure 13— Switch-over MOSFET Parasitic Diodes
Design Procedure
When designing a switch mode supply the input voltage
range, load current, switching frequency, and inductor
ripple current must be specified.
It is important to prevent forward bias of these diodes.
The following two conditions must be satisfied in order for
the parasitic diodes to stay off.
• VDDA > V_LDO
• VDDA > V_OUT
If either V_LDO or V_OUT is higher than VDDA, then the respec-
tive diode will turn on and the SC418 operating current
will then flow through this diode. This has the potential of
damaging the device.
Note that if the VDDA supply is 3.3V then both the LDO
and switching regulator outputs are limited to 3.3V
maximum. Trying to obtain higher voltages such as 5V
The maximum input voltage (V_INMAX) is the highest speci-
fied input voltage. The minimum input voltage ( V_INMIN) is
determined by the lowest input voltage after evaluating
the voltage drops due to connectors, fuses, switches, and
PCB traces.
The following parameters define the design.
• Nominal output voltage (V_OUT)
• Static or DC output tolerance
• Transient response
• Maximum load current (I_OUT)
There are two values of load current to evaluate — con-
tinuous load current and peak load current. Continuous
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