sc2449iswtrt Semtech Corporation, sc2449iswtrt Datasheet - Page 15

• Manufacturer Part Number: sc2449iswtrt
• Description: Sc2449 Bi-phase/dual Controller
• Manufacturer: Semtech Corporation

The switching frequency of each channel is one half of
the oscillator frequency. The oscillator clock is also avail-
able externally through the CLKOUT pin and can be used
to provide synchronization for other converters.
Bias Generation
A 6-7 Volt supply voltage is required to power up the
SC2449. This voltage could be provided by an external
power supply or derived from VIN through an external
pass transistor. REGDRV is the control signal to the base
of the pass transistor that will regulate VCC. The voltage
at the VCC pin is compared to the internal voltage refer-
ence, and the REGDRV pin can sink up to 5mA current to
regulate the voltage at the VCC pin.
Enable
If the ENABLE pin is connected to logic high, the SC2449
is enabled, while connecting it to ground will put the de-
vice into disabled mode. The ENABLE pin can also be
configured as input UVLO through input voltage divider
resistors. The controller will be enabled when the EN-
ABLE pin voltage reaches 2.05 V, and will be disabled
with 400mV hysteresis.
Under Voltage Lockout
Under Voltage lockout (UVLO) circuitry senses VCC through
a voltage divider. If this signal falls below 5.8V, with a
typical hysteresis of 400 mV, the BG pin is pulled low by
an internal transistor causing the lower MOSFET gate to
be on and the upper MOSFET gate off for both phases.
Over Voltage Protection
The SC2449 provides OVP protection for each output
individually. Once the converter output voltage exceeds
120% nominal output voltage, the lower MOSFET gates
are latched on and the upper MOSFET gates are latched
off. The latch is then reset once the OVP condition is
removed.
Soft Start
An external capacitor at the BG pin is used to set up the
Soft Start duration. The capacitor value, in conjunction
with the internal 3K resistor at the BG pin, control the
duration to bring up the bandgap to its final level. As the
Theory of Operation (Cont.)
POWER MANAGEMENT
2007 Semtech Corp.
15
BG capacitor is being charged through the internal resis-
tor, the PWM pulse width increases until the bandgap is
charged completely. This controlled start up of the PWM
prevents output voltage overshoot, unnecessary com-
ponent stress, and noise generation during start up.
Over Current Protection
The SC2449 current limit provides protection during an
over current condition. A sense resistor or PCB trace
can be used to sense the input supply current.
The over current protection trip point is determined by
the voltage drop across the sense resistor. Once this
voltage drop exceeds the voltage across the program-
ming resistor (50µA through R26), OCP protection circuit
will be triggered. Due to component and layout parasitics,
filtering might be necessary across the OC+ and OC- pins.
It is recommended to use a small RC filter with time con-
stant around 0.2µS. To clean up the phase node ringing,
one usually has to have a ceramic capacitor from the
top FET drain to the power ground. Too much capaci-
tance will bypass the top FET current from the sensing
resistor hence reducing OCP accuracy, while to little ca-
pacitance will not be able to clean up the phase node
ringing for full load operation. See application circuits.
Once an over current condition occurs, the lower MOSFET
gates are latched on and the upper MOSFET gates are
latched off. The latch is then reset at the beginning of
the next clock cycle. The cycle is repeated indefinitely
until the over current condition is removed.
Thermal Shutdown
In addition to current limit, the SC2449 monitors over
temperature condition. The over temperature detection
will shut down the part if the SC2449 die temperature
exceeds 150°C, and will auto reset once the die tem-
perature is dropped down.
Gate Drive
The SC2449 integrates high current gate drivers for fast
switching of large MOSFETs. The high-side gates can be
switched with peak currents of 1 Amp, while the larger
low-side gates can be switched with peak currents of 2
Amps. A cross conduction prevention circuitry ensures a
non-overlapping operation between the upper and lower
MOSFETs. This prevents false current limit tripping and
provides high efficiency.
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SC2449