SP6650 Datasheet, PDF (5/17 Pages) Sipex Corporation – High Efficiency 600mA Synchronous Buck Regulator Ideal for portable designs powered with Li Ion battery

OPERATION
The SP6650 is a synchronous buck regulator
with an input voltage range of +2.7V to +6.5V
and an output that is either preset to +3.3V, or
adjustable between +1.25V and V_IN. The SP6650
features a unique on-time control loop that runs
in discontinuous conduction mode (DCM) or
continuous conduction mode (CCM) using syn-
chronous rectification. Other features include
overtemperature shutdown, overcurrent protec-
tion, undervoltage lockout, digitally controlled
enable, a battery low indicator, and an external
feedback pin.
The SP6650 operates with a light load quiescent
current of 70µA using a 0.4Ω PMOS main
switch and a 0.3Ω NMOS auxiliary switch. It
operates with excellent efficiency across the
entire load range, making it an ideal solution for
battery powered applications and low current
step-down conversions. The part smoothly tran-
sitions into a 100% duty cycle under heavy load/
low input voltage conditions.
On-Time Control
The SP6650 uses a precision comparator and a
minimum on-time one-shot to regulate the out-
put voltage and control the inductor current
under normal load conditions. As the feedback
node (negative terminal of the loop comparator)
drops below the reference, the loop comparator
output goes high and closes the main switch.
The minimum on-time one shot is triggered,
setting a logic high for the duration defined by:
T_ON = ^KON
V_IN-V_OUT
where:
K_ON = 2.7µs*V constant
V_IN = V_IN pin voltage
V_OUT = V_OUT pin voltage
The outputs of the loop comparator and the on-
time one shot are OR'd together, inverted, and
buffered to drive the gate of the high side PMOS
main switch. Increasing inductor current causes
the output to increase through the ESR (equiva-
lent series resistance) of the output capacitor. As
V_OUT rises above the regulation threshold, the
loop comparator output resets low. Termination
of the on cycle occurs when both the loop
comparator and the on-time one shot goes to
logic low, or the inductor current limit has been
reached.
The discharge phase follows with the high side
PMOS switch opening and the low side NMOS
switch closing to provide a discharge path for
the inductor current. The decreasing inductor
current and the load current cause the output
voltage to droop. Under normal load conditions
when the inductor current is below the pro-
grammed limit, the off-time will continue until
the output voltage falls below the regulation
threshold, which initiates a new charge cycle via
the loop comparator.
The inductor current "floats" in continuous con-
duction mode. During this mode the inductor
peak current is below the programmed limit and
the valley current is above zero. This is to satisfy
load currents that are greater than half the mini-
mum current ripple. The current ripple, I_LR, is
defined by the equation:
I_LR ≈ ^{KON * VIN-VOUT-IOUT*Rch}
L
V_IN-V_OUT
where:
L = Inductor value
I_OUT = Load current
Rch = PMOS on resistance, 0.4Ω typ.
If the I_OUT*Rch term is negligible compared
with (V_IN-V_OUT), the above equation simplifies
to:
I_LR ≈ ^KON
L
For most applications, the inductor current ripple
controlled by the SP6650 is constant regardless
of input and output voltage. Because the output
voltage ripple is equal to:
where:
V_OUT(ripple) = I_LR*R_ESR
R_ESR = ESR of the output capacitor
the output ripple of the SP6650 regulator is
independent of the input and output voltages.
For battery powered applications, where the
battery voltage changes significantly, the SP6650
provides constant output voltage ripple through-
out the battery lifetime. This greatly simplifies
the LC filter design.
Date: 5/25/04
SP6650 High Efficiency 600mA Synchronous Buck Regulator
5
© Copyright 2004 Sipex Corporation