sc424evb Semtech Corporation, sc424evb Datasheet - Page 21

• Manufacturer Part Number: sc424evb
• Description: 6a Integrated Fet Regulator With 5v Ldo
• Manufacturer: Semtech Corporation

Applications Information (continued)
Note that it is possible to operate the switcher with the
LDO disabled, but the ENL pin must be below the logic
low threshold (0.4V maximum). The table below summa-
rizes the function of the ENL and EN pins, with respect to
the rising edge of ENL.
Figure 12 below shows the ENL voltage thresholds and
their eff ect on LDO and Switcher operation.
ENL Logic Control of PWM Operation
When the ENL input is driven above 2.6V, it is impossible to
determine if the LDO output is going to be used to power
the device or not. In self-powered operation where the
LDO will power the device, it is necessary during the LDO
start-up to hold the PWM switching off until the LDO has
reached 90% of the fi nal value. This prevents overloading
the current-limited LDO output during the LDO start-up.
However, if the switcher was previously operating (with
EN/PSV high but ENL at ground, and V5V supplied exter-
nally), then it is undesirable to shut down the switcher. To
prevent this, when the ENL input is above 2.6V (above the
V
high
high
high
IN
low
low
low
EN
UVLO threshold), the internal logic checks the PGOOD
2.6V
2.4V
ENL low
threshold
(min 0.4V)
AGND
high, < 2.6V
high, < 2.6V
high, > 2.6V
high, > 2.6V
low, < 0.4V
low, < 0.4V
ENL
Figure 12 — ENL Thresholds
ENL voltage
LDO on
Switcher on if EN = high
LDO on
Switcher off by V
LDO off
Switcher on if EN = high
LDO status
off
off
on
on
on
on
V
IN
UVLO hysteresis
IN
Switcher status
UVLO
off
on
off
off
off
on
signal. If PGOOD is high, then the switcher is already
running and the LDO will run through the start-up cycle
without aff ecting the switcher. If PGOOD is low, then the
LDO will not allow any PWM switching until the LDO
output has reached 90% of it’s fi nal value.
Using the On-chip LDO to Bias the SC414/SC424
The following steps must be followed when using the
internal LDO to bias 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 bias
and power output of the SC414. When V
configuration can cause problems due to the parasitic
diodes in the LDO switchover circuitry. After the Vout >
4.5V 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 specifi ed.
The maximum input voltage (V
fi ed input voltage. The minimum input voltage ( V
determined by the lowest input voltage after evaluating
the voltage drops due to connectors, fuses, switches, and
PCB traces.
The following parameters defi ne the design.
•
•
•
•
•
•
•
Connect V5V to VLDO before enabling the LDO.
Any external load on VLDO should not exceed
40mA until the LDO voltage has reached 90% of
fi nal value.
Do not connect the EN pin directly to the V5V or
any other supply voltage if Vout is greater than
or equal to 4.5V
Nominal output voltage (V
Static or DC output tolerance
Transient response
Maximum load current (I
OUT
INMAX
OUT
)
SC414/SC424
)
) is the highest speci-
OUT
> 4.5V this
INMIN
) is
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