sc4510 Semtech Corporation, sc4510 Datasheet - Page 10

• Manufacturer Part Number: sc4510
• Description: Sc4510 High-performance Synchronous Buck Controller With Reference Tracking
• Manufacturer: Semtech Corporation

INTRODUCTION
The SC4510 is designed to control and drive N-Channel
MOSFET synchronous rectified buck convertors. It has
additional features such as reference tracking, buffered
REFOUT that make it particularly attractive for tracking
output applications such as DDR memories. It can operate
the convertor in both the source and sink modes. The
switching frequency is programmable to optimize design.
The current mode switching regulator section features
Semtech’s patented Combi Sense technique for lossless
current sensing and provides a hiccup mode overcurrent
protection.
POWERING THE CONTROLLER
Supplies VIN, PVCC and AVCC from the input source are
used to power the SC4510. The AVCC and PVCC can be
tied to VIN supply or can be from separate source to
optimise efficiency. The AVCC supply provides the bias for
the oscillator, PWM switcher, voltage feedback, current
sense and the Power OK circuitry. PVCC is used to drive
the low and high side MOSFET gates. Minimum operating
limit for VIN is 2.5V typical. However, PVCC and AVCC have
higher UVLO limits as explained below. Maximum range
for all of the input and supply voltages is 16V.
Startup is inhibited until AVCC input reaches its UVLO
threshold . The UVLO limit is 4.5V typical. When AVCC is
below the UVLO threshold, the soft start pin is pulled low
and output drivers are turned off. The power up sequence
is initiated by a 2 uA current source charging the soft start
capacitor connected to the SS pin. When the SS pin reaches
0.5V, the convertor will start switching. The reference input
of the error amplifier is ramped up with the soft-start signal.
The soft start duration is controlled by the value of the SS
cap. The soft start pin also functions as an enabler with
TTL compatible input thresholds. If the SS/EN pin is pulled
below 0.6V, the SC4510 is disabled and draws very low
current.
REFERENCE INPUT AND BUFFERED OUTPUT
The SC4510 comes with a low level built in reference of
0.5V. The non inverting input of the error amplifier is
brought out to provide additional flexibility and output
tracking functions. In the basic operation, the REF and
REFIN pins may be shorted together and the output voltage
feedback is provided at IN
POWER MANAGEMENT
Functional Description
STARTUP AND ENABLE
2004 Semtech Corp.
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pin.
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In tracking applications such as DDR memories an external
reference may be brought in and applied at REFIN pin. The
range of applicable external voltage is up to 3V. The UV
and OV sensing thresholds for PGOOD output are centered
+12% around REFIN voltage.
The voltage at REFIN pin is buffered and put out as REFOUT.
This output tracks REFIN accurately within + 10 mV offset
or 0.5% of REFIN, whichever is higher. It can also source
up to 5 mA current. The REFOUT output is particularly useful
in DDR memory termination applications.
OSCILLATOR
The switching frequency F
external resistor using the following formula:
R
oscillator frequency is from 100 kHz to 1 MHz. The
maximum duty cycle available at any given frequency is
limited by minimum pulse width requirement which is
typically 250 nS. This gives a typical Dmax of 70% at the
highest frequency of 1 MHz or about 90% at 300 kHz.
The oscillator can be synchronised to an external clock that
is nominally faster than the internal frequency set by R
The synchronising signal should be TTL compatible, with
transitions above 2.0V and below 0.6V. The external voltage
level applied should be lower than AVCC of the device.
GATE DRIVERS
The low side gate driver is supplied from PVCC and provides
a peak source/sink current of 1A. The high side gate drive
is capable of sourcing and sinking peak currents of 0.75A.
Protection logic provides a typical dead time of 90 nS to
ensure both the upper and lower MOSFETs will not turn on
simultaneously and cause a shoot through condition.
The high side Mosfet gate drive can be derived from the
PVCC supply using the classical bootstrap technique as
illustrated in the applications circuits. A bootstrap capacitor
is connected from BST to the Phase node while PVCC is
connected through a low V
to the BST. This will provide a gate to source voltage
approximately equal to the (VCC - Vfwd).
OSC
R
OSC
is in k and F
= 66,000
OSC
/
F
is in kHz. The nominal range for the
OSC
OSC
F
Schottky or an ultrafast diode
of the SC4510 is set by an
www.semtech.com
SC4510
OSC.