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5
Rev F: 11/01/06 SP7652 Wide Input Voltage Range 6A, 600kHz, Buck Regulator © Copyright 2006 Sipex Corporation
UVLO fault is detected, the SP7652 is forced
into an idle state where the output drivers are
is attempted.
Soft Start
Soft start is achieved when a power con-
verter ramps up the output voltage while
controlling the magnitude of the input sup-
ply source current. In a modern step down
converter, ramping up the positive terminal
As a result, excess source current can be
the output capacitor.
IVin = COutOutSOft-StaRt
The SP7652 provides the user with the op-
tion to program the soft start rate by tying
selection of this capacitor is based on the
10uA pull up current present at the SS pin
and the 0.8V reference voltage. Therefore,
IVin = COutOut *10µA / (CSS
Under Voltage Lock Out (UVLO)
The SP7652 contains two separate UVLO
comparators to monitor the internal bias
(Vccin
independently. The Vcc UVLO threshold
is internally set to 4.25V, whereas the Vin
UVLO threshold is programmable through
the UVIN pin. When the UVIN pin is
greater than 2.5V, the SP7652 is permit-
ted to start up pending the removal of all
other faults. Both the Vcc and Vin UVLO
comparators have been designed with
hysteresis to prevent noise from resetting
a fault.
Thermal and Short-Circuit Protection
Because the SP7652 is designed to drive
large output current, there is a chance that the
power converter will become too hot. There-
has been included to prevent the IC from
malfunctioning at extreme te
mp
eratures.
A short-circuit detection comparator has
also been included in the SP7652 to protect
against an accidental short at the output
of the power converter. This comparator
constantly monitors the positive and nega-
the Vfb
below the positive reference, a short-circuit
fault is set. Because the SS pin overrides the
internal 0.8V reference during soft start, the
SP7652 is capable of detecting short-circuit
faults throughout the duration of soft start as
well as in regular operation.
Handling of Faults
-
mal, or short-circuit faults, the SP7652 is
forced into an idle state where the SS and
COMP pins are pulled low and the NFETS
are held off. In the event of UVLO fault, the
SP7652 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 attempted im-
mediately after the 200ms timeout expires.
Whereas, when a thermal fault is detected,
the 200ms delay continuously recycles and a
re-start cannot be attempted until the thermal
fault is removed and the timer expires.
Error Amplier and Voltage Loop
Since the heart of the SP7652 voltage error
loop is a high performance, wide bandwidth
transconductance amplifier, great care
should be taken to select the optimal compen-
there are many ways to compensate the
voltage loop or to control the COMP pin ex-
ternally. If a simple, single-pole, single-zero
response is desired, then compensation can
a more complex compensation is required,
to run Type III compensation schemes with
adequate gain and phase margins at cross-
THEORY OF OPERATION