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SC615AEVB Datasheet(PDF) 7 Page - Semtech Corporation

Part # SC615AEVB
Description  Charge Pump Flash LED Driver
Download  12 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC615AEVB Datasheet(HTML) 7 Page - Semtech Corporation

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© 2008 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC615A
Output Over-Voltage Protection
Output over-voltage protection is included to prevent the
SC615A from generating an output voltage that could dam-
age other devices connected to it such as load LEDs and
bypass capacitors. When the output voltage exceeds 5.5V,
the OVP circuit disables the charge pump until the voltage
decreases to a level within the acceptable range.
This circuit allows the device to drive LEDs with high forward
voltages at a reduced level without exceeding the output
voltage limits specified for the device. Note, however, that
this effect is a consequence of the OVP circuit and is not
its intended purpose.
Over-Temperature Protection
The over-temperature circuit helps prevent the device from
overheating and experiencing a catastrophic failure. When
the junction temperature exceeds 150°C the
device is disabled. It remains disabled until the junction
temperature drops below this threshold. As with the UVLO
and OVP circuits, hysteresis is included to prevent toggling
between modes.
Over-Current Protection
When the SC615A is in 2X charge pump mode, the input
current will be approximately double the required output.
When the steady-state load requires the maximum current
available in 2X mode, the OCP circuit prevents the device
from overheating due to excessive power dissipation.
Short-Circuit Current Protection
Short-circuit current protection is provided to limit the
current that can be sourced when the output is shorted
to ground. When a short circuit forces V
OUT to drop below
2V, the SCCP detects the condition and limits the output
current.
Resistor Selection
The ISET resistor selection is critical in generating the cor-
rect current. The value can be chosen to set the Spotlight
mode current or the Flash mode current, but it must be
noted that the two current settings are dependent on the
same resistor. It is recommended that the resistor be se-
lected to match the desired LED current for Flash mode.
This allows the designer to set the SC615A’s maximum cur-
rent and select the package size necessary for the power
dissipation required in Flash mode. The typical application
shown on page 1 uses a 0.5
Ω resistor to set a flash mode
current of 500mA and spotlight mode current of 200mA.
A high precision resistor should be used to ensure the
specified accuracy for LED current.
To avoid malfunction of the charge pump, it is recom-
mended that the resistance seen at the ISET pin remain
constant while the device is active (Flash or Spotlight
mode). Changing the resistance value or the load current
while the device is active could cause instability that would
result in non-compliant behavior.
The load current return path is from the ISET sense point
through the resistor and back to the ground pins. Resis-
tance in this path adds to the total resistance and has the
effect of reducing the LED current by about 4% per 10mV
of DC drop across the return copper trace in flash mode.
For this reason, it is crucial to have a low resistance return
path. Place and ground the resistor as close as possible
to the ground pin of the SC615A. The trace from the ISET
pin has virtually no current. The ISET trace should make
contact at the pad of the power sense resistor to minimize
the effect of voltage drop between the LED cathode and
the resistor.
Capacitor Selection
The SC615A is designed to use low-ESR ceramic capaci-
tors for the input and output bypass capacitors as well as
the charge pump bucket capacitors. Ideal performance
is achieved when C1 is exactly equal to C2. It is recom-
mended that X5R or X7R ceramic capacitors be used for
best performance.
Thermal Resistance
The SC615A package is thermally efficient when the circuit
board layout connects the thermal pad though multiple
vias to the ground plane. The thermal resistance is rated
at 49°C/W and this rating is dependent on the connection
between the thermal pad and the ground plane. A layout
that is done correctly should keep the junction tempera-
ture below the OT limit while operating the SC615A within
the specified electrical conditions for I
OUT and VISET.
A poor
layout may allow the junction temperature to reach the
OT limit while in Flash or Spotlight mode. So, it is critical
to maintain adequate ground plane around the device
to maximize heat transfer and avoid over-temperature
shutdown.
Applications Information (Cont.)


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