SC414MLTRT Semtech, SC414MLTRT Datasheet - Page 20

• Manufacturer Part Number: SC414MLTRT
• Description: IC BUCK SYNC ADJ 6A 28MLPQ
• Manufacturer: Semtech
• Series: SmartDrive™r
• Type: Step-Down (Buck)r

Specifications of SC414MLTRT

• Mfg Application Notes: SC414 Basic Design Procedure
• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 5.5 V
• Current - Output: 6A
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC414MLTR

Applications Information (continued)
LDO Switch-Over Operation
The SC414/SC424 includes a switch-over function for the
LDO. The switch-over function is designed to increase
effi ciency by using the more effi cient DC-DC converter to
power the LDO output, avoiding the less efficient LDO
regulator when possible. The switch-over function con-
nects the VLDO pin directly to the VOUT pin using an
internal switch. When the switch-over is complete the
LDO is turned off , which results in a power savings and
maximizes effi ciency. If the LDO output is used to bias the
SC414/SC424, then after switch-over the device is self-
powered from the switching regulator with the LDO
turned off .
The switch-over logic waits for 32 switching cycles before
it starts the switch-over. There are two methods that
determine the switch-over of V
In the fi rst method, the LDO is already in regulation and
the DC-DC converter is later enabled. As soon as the
PGOOD output goes high, the 32 cycle counter is started.
The voltages at the VLDO and VOUT pins are then com-
pared; if the two voltages are within ±300mV (typically) of
each other, within 32 cylces, the VLDO pin connects to the
VOUT pin using an internal switch, and the LDO is turned
off .
In the second method, the DC-DC converter is already
running and the LDO is enabled. In this case the 32 cycles
are started as soon as the LDO reaches 90% of its fi nal
value. At this time, the VLDO and VOUT pins are compared,
and if within ±300mV (typically) the switch-over occurs
and the LDO is turned off .
Switch-over Limitations on VOUT and VLDO
Because the internal switch-over circuit always compares
the VOUT and VLDO pins at start-up, there are voltage
limitations on permissible combinations of these pins.
Consider the situation where V
After start-up, the device would connect VOUT to VLDO
and disable the LDO, since the two voltage are within the
±300mV switch-over window. To avoid unwanted switch-
over, the minimum diff erence between the voltages for
V
OUT
and V
LDO
should be ±500mV.
OUT
LDO
is programmed to 4.7V.
to V
OUT
.
Switch-over MOSFET Parasitic Diodes
The switch-over MOSFET contains parasitic diodes that
are inherent to its construction, as shown in Figure 11.
There are some important design rules that must be fol-
lowed to prevent forward bias of these diodes. The fol-
lowing two conditions need to be satisfi ed in order for the
parasitic diodes to stay off .
If either V
diode will turn on and the SC414/SC424 operating current
will flow through this diode. This has the potential of
damaging the device.
ENL pin and V
The ENL pin also acts as the switcher under-voltage
lockout for the V
grammable via a resistor divider at the VIN, ENL, and AGND
pins.
ENL is the enable/disable signal for the LDO. In order to
implement the V
that needs to be satisfi ed.
If the ENL pin transitions low within 2 switching cycles and
is < 1V, then the LDO will turn off but the switcher remains
on. If ENL goes below the V
above 1V, then the switcher will turn off but the LDO
remains on.
The V
the V
•
•
Figure 11— Switch-over MOSFET Parasitic Diodes
IN
V5V ≥ V
V5V ≥ V
IN
rising edge. The falling edge threshold is 2.4V.
UVLO function has a typical threshold of 2.6V on
LDO
or V
Parasitic diode
V
Switchover
LDO
OUT
LDO
control
IN
OUT
IN
IN
UVLO there is also a timing requirement
UVLO
supply. The V
is higher than V5V, then the respective
V5V
IN
UVLO threshold and stays
Switchover
MOSFET
IN
SC414/SC424
Parasitic diode
UVLO voltage is pro-
V
OUT
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