SP6134 Datasheet, PDF (6/14 Pages) Sipex Corporation – Dual Supply Synchronous Buck Controller

tection features include thermal shutdown and
short-circuit detection. In the event that either a
thermal, short-circuit, or UVLO fault is de-
tected, the SP6134 is forced into an idle state
where the output drivers are held off for a finite
period before a re-start is attempted.
Under Voltage Lock Out (UVLO)
The SP6134 contains two separate UVLO com-
parators to monitor the bias (V_CC) and conver-
sion (V_IN) voltages independently. The V_CC
UVLO threshold is internally set to 4.25V,
whereas the V_IN UVLO threshold is program-
mable through the UVIN pin. When the UVIN
pin is greater than 2.5V, the SP6134 is permitted
to start up pending the removal of all other
faults. Both the V_CC and V_IN UVLO compara-
tors have been designed with hysteresis to pre-
vent noise from resetting a fault.
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.
IV_IN = C_OUT * DV_OUT / DTSoft-start
The SP6134 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:
IV_IN = C_OUT * DV_OUT *10µA / (C_SS * 0.8V)
THEORY OF OPERATION: Continued
Hiccup
Upon the detection of a power, thermal, or short-
circuit fault, the SP6134 is forced into an idle
state for 100mS (typical). The SS and COMP
pins are immediately pulled low, and the gate
drivers are held off for the duration 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 100mS (typi-
cal) as long as the short-circuit condition per-
sists.
Thermal and Short-Circuit
Protection
Because the SP6134 is designed to drive large
NFETs running at high current, there is a chance
that either the controller or power converter will
become too hot. Therefore, an internal thermal
shutdown (145°C) has been included to prevent
the IC from malfunctioning at extreme tempera-
tures.
A short-circuit detection comparator has also
been included in the SP6134 to protect against
the accidental short or sever build up of current
at the output of the power converter. This com-
parator constantly monitors the positive and
negative terminals of the error amplifier, and if
the V_FB pin ever falls more than 250mV (typi-
cal) below the positive reference, a short-circuit
fault is set. Because the SS pin overrides the
internal 0.8V reference during soft start, the
SP6134 is capable of detecting short-circuit
faults throughout the duration of soft start as
well as in regular operation.
Error Amplifier and Voltage Loop
As stated before, the heart of the SP6134 voltage
error loop is a high performance, wide band-
width transconductance amplifier. Because of
the amplifier’s current limited (+/-150µA)
transconductance, there are many ways to com-
pensate the voltage loop or to control the COMP
pin externally. If a simple, single pole, single
Date: 5/25/04
SP6134 Dual Supply, Synchronous Buck Controller
6
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