SC403B Datasheet, PDF (22/32 Pages) Semtech Corporation – 6A EcoSpeed® Integrated FET Regulator with Programmable LDO

SC403B
Applications Information (continued)
In many applications, the EN/PSV pin will be pulled high to
the VDD node to allow control of the PWM and LDO ENL
pin. If the switch over feature is used, this circuit must be
implemented with caution or the circuit may be damaged.
In the case where the ENL pin is being controlled by a GPIO
signal or is tied directly to the input voltage, the ENL pin can
be pulled low while the PWM is still generating an output
voltage that is seen across one of the switch-over diodes.
This may result in the VDD node being held above its UVLO
threshold while the LDO is deactivated. This type of opera-
tion can potentially damage the part.
In the case where the ENL pin is used to control the input
UVLO, it is acceptable to connect EN/PSV directly to the
VDD node.
It is not recommended to use the switch-over feature for
an output voltage less than 3V since this does not provide
sufficient voltage for the gate-source drive to the internal
p-channel switch-over MOSFET.
Switch-over MOSFET Parasitic Diode
The switch-over MOSFET contains a parasitic diode that is
inherent to its construction, as shown in Figure 12.
Switchover
control
LDO
Switchover
MOSFET
V_OUT
Parasitic diode
VDD
the bias and power output of the SC403B. When V_OUT >
3.0V this configuration can cause problems due to the
parasitic diodes in the LDO switchover circuitry. After the
V_OUT > 3.0V PWM output is up and running the switchover
diodes can hold up V5V > UVLO even if the ENL pin is
grounded, turning off the LDO. Operating in this way can
potentially damage the part.
Design Procedure
When designing a switch mode supply the input voltage
range, load current, switching frequency, and inductor
ripple current must be specified.
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
load current relates to thermal stresses which drive the
selection of the inductor and input capacitors. Peak load
current determines instantaneous component stresses and
filtering requirements such as inductor saturation, output
capacitors, and design of the current limit circuit.
Figure 12— Switch-over MOSFET Parasitic Diodes
If V_OUT is higher than VDD, then the diode will turn on and
the SC403B operating current will flow through this diode.
This has the potential of damaging the device.
There are some important design rules that must be fol-
lowed to prevent forward bias of this diode. The following
condition, V_DD ≥ V_OUT needs to be satisfied in order for the
parasitic diode to stay off and prevent damaging the
device. Many applications connect the EN pin to V5V and
control the on/off of the LDO and PWM simultaneously
with the ENL pin. This allows one signal to control both
The following values are used in this design.
•
•
•
•
V_IN = 12V + 10%
V_OUT = 1.5V + 4%
f_SW = 300kHz
Load = 6A maximum
• V_FB = 600mV
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