SP6138 Datasheet, PDF (6/17 Pages) Sipex Corporation – Synchronous Buck Controller

THEORY OF OPERATION
The SP6138 contains two unique control
features that are very powerful in distributed
applications. First, non-synchronous driver
control is enabled during start up to prohibit
the low side NFET from pulling down the out-
put until the high side NFET has attempted to
turn on. Second, a 100% duty cycle timeout
ensures that the low side NFET is periodically
enhanced during extended periods at 100%
duty cycle. This guarantees the synchronized
refreshing of the BST capacitor during very
large duty ratios.
The SP6138 also contains a number of valu-
able protection features. A programmable
input UVLO allows a user to set the exact
value at which the conversion voltage is at a
safe point to begin down conversion, and an
internal V_CC UVLO ensures that the controller
itself has enough voltage to properly operate.
Other protection features include thermal
shutdown and short-circuit detection. In the
event that either a
thermal, short-circuit, or UVLO fault is de-
tected, the SP6138 is forced into an idle state
where the output drivers are held off for a
finite period before a re-start 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 non-inverting input
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
IV_{IN, X}
=
Cout • ∆Vout
∆TSoft-start
The SP6138 provides the user with the op-
tion 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
10µA pull up current present at the SS pin
and the 0.8V reference voltage. Therefore,
the excess current source can be redefined as:
I V_{IN, X} = C_{o ut} • ∆V_out •
Hiccup
10µA
(C_ss • 0.8V)
Upon the detection of a power, thermal, or
short-circuit fault, the SP6138 is forced into
an idle state for a minimum of 30ms. The SS
and COMP pins are immediately pulled low,
and the gate drivers are held off for the dura-
tion of the timeout period. Power and thermal
faults have to be removed before a restart
may be attempted, whereas, a short-circuit
fault is internally cleared shortly after the fault
latch is set. Therefore, a restart attempt is
guaranteed every 30ms (typical) as long as
the short-circuit condition persists.
A short-circuit detection comparator has
also been included in the SP6138 to protect
against the accidental short or severe build
up of current at the output of the power con-
verter. This comparator constantly monitors
the inputs to theerror amplifier, and if the
V_FB pin ever falls more than 250mV (typical)
below the reference voltage, a short-circuit
fault is set. Because the SS pin overrides the
internal 0.8V reference during soft start, the
SP6138 is capable of detecting short-circuit
faults throughout the duration of soft start as
well as in regular operation.
Error Amplifier & Voltage Loop
As stated before, the heart of the SP6138
voltage error loop is a high performance,
wide bandwidth transconductance ampli-
fier. Because of the amplifier’s current
limited (+100µA) 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 required, then compensation can be as
simple as an RC circuit to ground. If a more
complex compensation is required, then the
amplifier has enough bandwidthto run Type III
compensation schemes with adequate gain
and phase margins at crossover frequencies
greater than 200 kHz.
Oct 24-06 Rev J
SP6138 Synchronous Buck Controller

© 2006 Sipex Corporation