SP6120 Datasheet, PDF (9/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

driver as well as whether the controller is oper-
ating in continuous or discontinuous mode. The
Fault Logic holds the high and low side drivers
off if V_CC dips below 2.75V, if an over current
condition exists, or if the part is disabled through
the ENABLE pin. The Zero Crossing Compara-
tor turns the lower driver off if the conduction
current reaches zero and the Driver Logic has
made an attempt to turn the lower driver on and
the Programming Logic is set for discontinuous
mode. Lastly, the 4Ω drivers have internal gate
non-overlap circuitry and are designed to drive
MOSFETs associated with converter designs in
the 5A to 10A range. Typically the high side
driver is referenced to the SWN pin; further
improving the efficiency and performance of
the converter.
ENABLE
Low quiescent mode or “Sleep Mode” is initi-
ated by pulling the ENABLE pin below 650mV.
The ENABLE pin has an internal 4µA pull-up
current and does not require any external inter-
face for normal operation. If the ENABLE pin
is driven from a voltage source, the voltage must
be above 1.45V in order to guarantee proper
“awake” operation. Assuming that V_CC is above
2.85V, the SP6120 transitions from “Sleep
Mode” to “Awake Mode” in about 20µs to 30µs
and from “Awake Mode” to “Sleep Mode” in a
few microseconds. SP6120 quiescent current in
sleep mode is 20µA maximum. During Sleep
Mode, the high side and low side MOSFETs are
turned off and the COMP and SS pins are held
low.
UVLO
Assuming that the ENABLE pin is either pulled
high or floating, the voltage on the V_CC pin then
determines operation of the SP6120. As V_CC
rises, the UVLO block monitors V_CC and keeps
the high side and low side MOSFETs off and the
COMP and SS pins low until V_CC reaches 2.85V.
If no faults are present, the SP6120 will initiate
a soft start when V_CC exceeds 2.85V. Hysteresis
(about 100mV) in the UVLO comparator pro-
vides noise immunity at start-up.
THEORY OF OPERATIONS: Continued
Soft Start
(see figures on next page)
Soft start is required on step-down controllers to
prevent excess inrush current through the power
train during start-up. Typically this is managed
by sourcing a controlled current into a program-
ming capacitor (on the SS pin) and then using
the voltage across this capacitor to slowly ramp
up either the error amp reference or the error
amp output (COMP). The control loop creates
narrow width driver pulses while the output
voltage is low and allows these pulses to in-
crease to their steady-state duty cycle as the
output voltage increases to its regulated value.
As a result of controlling the inductor
volt*second product during start-up, inrush cur-
rent is also controlled.
The presence of the output capacitor creates
extra current draw during start-up. Simply stated,
dV_OUT/dt requires an average sustained current
in the output capacitor and this current must be
considered while calculating peak inrush cur-
rent and over current thresholds. Since the
SP6120
Soft Start: (continued)
ramps up the error amp reference voltage, an
expression for the output capacitor current can
be written as:
IC_OUT = (C_OUT/C_SS) * (V_OUT/1.25) * 50µA
As the figure shows, the SS voltage controls a
variety of signals. First, provided all the exter-
nal fault conditions are removed, the fault latch
is reset and the SS cap begins to charge. When
the SS voltage reaches around 0.3V, the error
amp reference begins to track the SS voltage
while maintaining the 0.3V differential. As the
SS voltage reaches 0.7V, the driver begins to
switch the high side and low side MOSFETs
with narrow pulses in an effort to keep the
converter output regulated. As the error amp
reference ramps upward, the driver pulses widen
until a steady state value is reached. The “bump”
in the inductor current transfer curve is indica-
tive of excess charge current incurred due to the
Date: 5/25/04
SP6120 Low Voltage, AnyFET^TM, Synchronous, Buck Controller
9
© Copyright 2004 Sipex Corporation