SC461 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information (continued)
threshold and stays above 1V, then the switcher will turn
off but the LDO will remain on.
additional current needed by the DH and DL gate drives
from overloading the LDO at start-up.
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), otherwise the VIN UVLO
function will disable the switcher.
LDO Start-up
Before LDO start-up, the device checks the status of the
following signals to ensure proper operation can be
maintained.
The next table summarizes the function of the ENL and EN
pins.
EN
ENL
LDO status Switcher status
low low, < 0.4V
off
off
high low, < 0.4V
off
on
low high, < 1.24V
on
off
high high, < 1.24V
on
off
low high, > 1.55V
on
off
high high, > 1.55V
on
on
Figure 12 shows the ENL voltage thresholds and their
effect on LDO and Switcher operation.
ENL voltage
LDO on
Switcher on if EN = high
1.55V
1.24V
ENL low
threshold
(min 0.4V)
AGND
VIN UVLO hysteresis
LDO on
Switcher off by VIN UVLO
LDO off
Switcher on if EN = high
Figure 12 — ENL Thresholds
ENL Logic Control of PWM Operation
When the ENL input exceeds the VIN UVLO threshold of
1.55V, internal logic checks the PGOOD signal. If PGOOD
is high, the switcher is already running and the LDO will
start without affecting the switcher. If PGOOD is low, the
device disables PWM switching until the LDO output has
reached 80% of its final value. This delay prevents the
1. ENL pin
2. VLDO output
3. VIN input voltage
When the ENL pin is high and VIN voltage is available, the
LDO will begin start-up. During the initial phase when
VLDO is below 1V, the LDO initiates a current-limited start-
up (typically 15mA). This protects the LDO from thermal
damage if the VLDO pin is shorted to ground. As VLDO
exceeds 1V, the start-up current gradually increases to
96mA. When VLDO reaches 4V, the LDO current limit
increases to 200mA and the LDO output rises quickly to
5V. The LDO start-up profile is shown in Figure 13.
VLDO
5V
4V
voltage regulating with
200mA current limit
increasing current
1V
15mA constant current
Figure 13 — LDO Start-Up
Using the Internal LDO to Bias the SC461
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.
• During the initial start-up the LDO, when the
LDO output is less than 1V, the external load
should not exceed 10mA. Above 1V, any exter-
nal load on VLDO should not exceed 40mA until
the LDO voltage has reached 4V.
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