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SP6650 Datasheet(PDF) 6 Page - Sipex Corporation

Part No. SP6650
Description  High Efficiency 600mA Synchronous Buck Regulator Ideal for portable designs powered with Li Ion battery
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Maker  SIPEX [Sipex Corporation]
Homepage  http://www.sipex.com
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SP6650 Datasheet(HTML) 6 Page - Sipex Corporation

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Date: 5/25/04
SP6650 High Efficiency 600mA Synchronous Buck Regulator
© Copyright 2004 Sipex Corporation
RDS(ON) of the P-Channel MOSFET and RL is
the DC resistance of the inductor.
The on-time control circuit seamlessly operates
the converter between CCM, DCM, and low
dropout modes without the need for compensa-
tion. The converter's transient response is quick
since there is no compensated error amplifier in
the loop.
Inductor Over-Current Protection
The inductor over-current protection circuitry is
programmed to limit the peak inductor current
to 950mA (pin 4 tied to VIN) or 500mA (pin 4 to
ground). This is done during the on-time by
comparing the source to drain voltage drop of
the PMOS passing the inductor current with a
second voltage drop representing the maximum
allowable inductor current. As the two voltages
become equal, the over-current comparator trig-
gers a minimum off-time one shot. The off-time
one shot forces the loop into the discharge phase
for a minimum time causing the inductor current
to decrease.
At the end of the off-time loop, control is handed
back to the OR'd 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 over-
load conditions, the converter behaves like a
current source at the programmed limit minus
half the current ripple. The minimum TOFF is
6
µs (typ.) at VOUT = 0V and 2µs (typ.) for VOUT
greater than 1.5V.
Under-Voltage Lockout
The SP6650 is equipped with under-voltage
lockout to protect the input battery source from
excessive currents when substantially dis-
charged. When the input supply is below the
UVLO threshold both power switches are open
to prevent inductor current from flowing. The
internal reference and regulator circuitry are
enabled drawing the 70
µA light load quiescent
current on pin 2. The rising input voltage UVLO
threshold is +2.7V, with a typical hysteresis of
120mV to prevent chattering due to the imped-
ance of the input source.
On-Time Control: continued
The maximum loop frequency in CCM is de-
fined by the equation:
FLP
(VIN-VOUT)*(VOUT+IOUT*Rdc)
KON*[V
IN+IOUT*(Rdc-Rch)]
where:
FLP = CCM loop frequency
Rdc = NMOS on resistance, 0.3
Ωtyp.
Ignoring conduction losses simplifies the loop
frequency to
FLP =
1
*
VOUT
* (VIN-VOUT)
KON
VIN
OR'ing the loop comparator and the on-time one
shot reduces the switching frequency for load
currents below half the inductor ripple current.
This increases light load efficiency. The mini-
mum on-time insures that the inductor current
ripple is a minimum of KON/L, more than the
load current demands. The converter goes in to
a standard pulse frequency modulation (PFM)
mode where the switching frequency is propor-
tional to the load current.
Low Dropout and Load Transient
Operation
OR'ing the loop comparator also increases the
duty ratio past the ideal D=VOUT/VIN up to and
including 100%. Under a light to heavy load
transient, the loop comparator will hold the
main switch on past the on-time one shot pulse
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, causing the loop compara-
tor to hold the converter 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 (VIN - VOUT) below
which the output regulation cannot be main-
tained. The dropout voltage of SP6650 is equal
to IL (0.4Ω + RL) where 0.4Ω is the typical


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