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SC1404ITSTR Datasheet(PDF) 17 Page - Semtech Corporation

Part # SC1404ITSTR
Description  Mobile Multi-Output PWM Controller with Virtual Current SenseTM
PDF  25 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1404ITSTR Datasheet(HTML) 17 Page - Semtech Corporation

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17
 2004 Semtech Corp.
www.semtech.com
SC1404
POWER MANAGEMENT
5V Start-up with slow Vin ramp.
Proper startup of the 5V output can be hampered by slow dV/dt on
the input. The SC1404 will power up and attempt to generate an
output when the input voltage exceeds 4.5 volts. If the input has
a slow dV/dt, the input voltage will not rise significantly during the
start-up sequence, leading to two conditions. First, the VL supply
can be hundreds of mV below 5V, since the input may not yet be
above 5V. Second, the duty cycle will be at maximum, leading to
very small off-times. These two conditions tend to reduce the
boost voltage; if continued indefinitely, the boost capacitor may
be unable to recharge fully, and eventually the high-side driver
loses its boost bias.
To avoid this the following steps should be taken:
1. If possible the dV/dt of the input supply should exceed .02V/
usec. This dV/dt condition only applies when the input passes
between 4 and 6 volts, the point at which the SC1404 begins a
startup sequence. An alternative is to make sure the input voltage
reaches 6 volts within 100 usec of SC1404 startup at
approximately 4.2 volts. This is sufficiently fast to allow VL and
duty cycle to achieve normal levels and prevents the boost voltage
from falling.
2. If the input dV/dt cannot meet condition 1, the startup of the
SC1404 should be delayed until the input voltage reaches 6V.
This can be done using either the SHDN# or ON5 pin. If the dV/dt
is moderate (slews from 4 to 6 volts in several msecs), an RC
delay on either the SHDN# or ON5 pin should be enough to delay
turn-on until the input reaches 6V.
3. For slow dV/dt on the input (10’s of msec), the SC1404 should
be held off until the input reaches 6V. This can be done using a
comparator or external logic to hold the SHDN# or ON5 pin low
until the input reaches 6V.
12V Load Limitations
The 12V regulator derives input power from a secondary winding
on the 5V inductor. During the 5V off-time, the inductor transfers
energy from the 5V winding to the secondary winding, thereby
providing a crudely regulated 15V that feeds the 12V regulator.
Note that duty cycle increases at low input voltages, and therefore
the on-time decreases. At low input voltages, the duty cycle
increases to maintain the 5V output. The off-time consequently
decreases, which has two detrimental effects. It allows less time
to recharge the raw 15V capacitor, and it also raises the peak 15V
current required to maintain the average 12V load. The 15V
winding needs higher peak current, delivered in less time. But the
stray (leakage) inductance of the inductor resists rapid changes in
winding current, and ultimately limits how much current can be
drawn from 15V before the voltage falls.
The following guidelines for 12V loading apply to the typical circuit,
page 22.
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