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SP7650 Fiches technique(PDF) 5 Page - Sipex Corporation

No de pièce SP7650
Description  Wide Input Voltage Range 3A, 300kHz, Buck Regulator
Download  16 Pages
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Fabricant  SIPEX [Sipex Corporation]
Site Internet  http://www.sipex.com
Logo SIPEX - Sipex Corporation

SP7650 Fiches technique(HTML) 5 Page - Sipex Corporation

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Date: /5/06
SP7650 Wide Input Voltage Range 3A, 300kHz, Buck Regulator
© Copyright 2006 Sipex Corporation
5
Date: 2/16/06
SP7650 Wide Input Voltage Range 3A, 300kHz, Buck Regulator
© Copyright 2006 Sipex Corporation
features include thermal shutdown and short-
circuit detection. In the event that either a ther-
mal, short-circuit, or UVLO fault is detected,
the SP7650 is forced into an idle state where the
output drivers are held off for a finite period
before a restart is attempted.
Soft Start
“Soft Start” is achieved when a power converter
ramps up the output voltage while controlling
the magnitude of the input supply source cur-
rent. In a modern step down converter, ramping
up the positive terminal of the error amplifier
controls soft start. As a result, excess source
current can be defined as the current required to
charge the output capacitor.
IVIN = COUT * ( VOUT / TSOFT-START)
The SP7650 provides the user with the option to
program the soft start rate by tying a capacitor
from the SS pin to GND. The selection of this
capacitor is based on the 10uA pull up current
present at the SS pin and the 0.8V reference
voltage. Therefore, the excess source can be
redefined as:
IVIN = COUT * ( VOUT *10µA / (CSS * 0.8V)
Under Voltage Lock Out (UVLO)
The SP7650 contains two separate UVLO com-
parators to monitor the bias (VCC) and conver-
sion (VIN) voltages independently. The VCC
UVLO threshold is internally set to 4.25V,
whereas the VIN UVLO threshold is program-
mable through the UVIN pin. When the UVIN
pin is greater than 2.5V, the SP7650 is permitted
to start up pending the removal of all other
faults. Both the VCC and VIN UVLO compara-
tors have been designed with hysteresis to pre-
vent noise from resetting a fault.
Thermal and Short-Circuit
Protection
Because the SP7650 is designed to drive large
output current, there is a chance that the power
converter will become too hot. Therefore, an
internal thermal shutdown (145°C) has been
included to prevent the IC from malfunctioning
at extreme temperatures.
A short-circuit detection comparator has also
been included in the SP7650 to protect against
an accidental short at the output of the power
converter. This comparator constantly monitors
the positive and negative terminals of the error
amplifier, and if the VFB pin falls more than
250mV (typical) below the positive reference, a
short-circuit fault is set. Because the SS pin
overrides the internal 0.8V reference during soft
start, the SP7650 is capable of detecting short-
circuit faults throughout the duration of soft
start as well as in regular operation.
Handling of Faults:
Upon the detection of power (UVLO), thermal,
or short-circuit faults, the SP7650 is forced into
an idle state where the SS and COMP pins are
pulled low and both switches are held off. In the
event of UVLO fault, the SP7650 remains in this
idle state until the UVLO fault is removed.
Upon the detection of a thermal or short-circuit
fault, an internal 200ms timer is activated. In the
event of a short-circuit fault, a re-start is at-
tempted immediately after the 200ms timeout
expires. Whereas, when a thermal fault is de-
tected the 200ms delay continuously recycles
and a re-start cannot be attempted until the
thermal fault is removed and the timer expires.
Error Amplifier and Voltage Loop
Since the heart of the SP7650 voltage error loop
is a high performance, wide bandwidth transcon-
ductance amplifier. Because of the amplifier’s
current limited (+/-150µA) transconductance,
there are many ways to compensate the voltage
loop or to control the COMP pin externally. If
a simple, single pole, single zero response is
THEORY OF OPERATION


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