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Date: 3/6/06 SP6656, 400mA Synchronous Buck Regulator with Dynamically Adjustable Voltage Output © Copyright 2006 Sipex Corporation
Low Dropout and Load Transient
Operation
Using AND logic function the loop compara-
tor also increases the duty ratio past the
ideal D= V
OUT
/V
IN
up to and including 100%.
Under a light to heavy load transient, the
loop comparator will hold the main switch
on longer than the minimum on timer until
the output is brought back into regulation.
Also, as the input voltage supply drops
down close to the output voltage, the main
MOSFET resistance loss will dictate a much
higher duty ratio to regulate the output.
Eventually as the input voltage drops low
enough, the output voltage will follow, caus-
ing the loop comparator to hold the con-
verter at 100% duty cycle.
This mode is critical in extending battery life
when the output voltage is at or above the
minimum usable input voltage. The dropout
voltage is the minimum (V
IN
-V
OUT
) below
which the output regulation cannot be main-
tained. The dropout voltage of SP6656 is
equal to I
L
* (0.3Ω+ R
L1
) where 0.3Ω is the
typical R
DS(ON)
of the P-Channel MOSFET
and R
L
is the DC resistance of the inductor.
The SP6656 has been designed to operate
in dropout with a light load Iq of only 80µA.
The on-time control circuit seamlessly op-
erates the converter between CCM, DCM,
and low dropout modes without the need for
compensation. The converter’s transient
response is quick since there is no compen-
sated error amplifier in the loop.
Inductor Over-Current Protection
To reduce the light load dropout Iq, the
SP6656 over-current system is only en-
abled when I
L1
> 400mA. The inductor over-
current protection circuitry is programmed
to limit the peak inductor current to 0.625A.
This is done during the on-time by compar-
ing the source to drain voltage drop of the
PMOS passing the inductor current with a
second voltage drop representing the maxi-
mum allowable inductor current. As the two
voltages become equal, the over-current
comparator triggers a minimum off-time one
shot. The off-time one shot forces the loop
into the discharge phase for a minimum
T
OFF
time causing the inductor current to
decrease. At the end of the off-time, loop
control is handed back to the AND logic
function on-time signal. If the output voltage
is still low, charging begins until the output
is in regulation or the current limit has been
reached again. During startup and overload
conditions, the converter behaves like a
current source at the programmed limit mi-
nus half the current ripple. The minimum
T
OFF
is controlled by the equation:
T
OFF (MIN)
=
KOFF
V
OUT
Under-Voltage Lockout
The SP6656 is equipped with a program-
mable under-voltage lockout to protect the
input battery source from excessive cur-
rents when substantially discharged. When
the input supply is below the UVLO thresh-
old both power switches are open to pre-
vent inductor current from flowing. The level
of falling input voltage has a typical hyster-
esis of 120mV to prevent chattering due to
the impedance of the input source.
Under-Current Detection
The synchronous rectifier is comprised of
an inductor discharge switch, a voltage com-
parator, and a driver latch. During the off-
time, positive inductor current flows into the
PGND pin 9 through the low side NMOS
switch to LX pin 10, through the inductor
and the output capacitor, and back to pin 9.
The comparator monitors the voltage drop
across the discharge NMOS. As the induc-
tor current approaches zero, the channel
voltage sign goes from negative to positive,
causing the comparator to trigger the driver
latch and open the switch to prevent induc-
tor current reversal. This circuit along with
the on-timer puts the converter into PFM
mode and improves light load efficiency
THEORY OF OPERATION
