sc2608 Semtech Corporation, sc2608 Datasheet - Page 6

• Manufacturer Part Number: sc2608
• Description: Sc2608 Simple, Synchronous Voltage Mode Pwm Controller
• Manufacturer: Semtech Corporation

Synchr
Synchr
U U U U U nder V
POWER MANAGEMENT
Theory of Operation
Synchr
Synchronous Buck Con
Synchr
The output voltage of the synchronous converter is set and
controlled by the output of the error amplifier. The invert-
ing input of the error amplifier receives its voltage from the
SENSE pin. The non-inverting input of the error amplifier is
connected to an internal 0.8V reference. The error ampli-
fier output is connected to the compensation pin. The
error amplifier generates a current proportional to (Vsense
- 0.8V), which is the COMP pin output current
(Transconductance ~ 7mS).
pin is the integral of the error amplifier current. The COMP
voltage is the non-inverting input of the PWM comparator
and controls the duty cycle of the MOSFET drivers. The
compensation network controls the stability and transient
response of the regulator. The larger the capacitor, the
slower the COMP voltage changes, and the slower the duty
cycle changes.
The non-inverting input voltage of the PWM comparator is
the triangular ramp signal generated from the oscillator.
The peak-to-peak voltage of the ramp is 1V, this is a pa-
rameter used in control loop calculation. When the oscil-
lator ramp signal rises above the COMP voltage, the com-
parator output goes high and the PWM latch is reset. This
pulls DH low, turning off the high-side MOSFET. After a
short delay (dead time), DL is pulled high, turning on the
low-side MOSFET. The oscillator also produces a set pulse
for the PWM latch to turn off the low-side MOSFET, After a
delay time, DH is pulled high to turn on the high-side
MOSFET. The delay time is determined by a monostable
on the chip.
The triangle wave minimum is about 0.8V, and the maxi-
mum is about 1.8V. Thus, if Vcomp = 0.7V, high side duty
cycle is the minimum (~0%) , but if Vcomp is 1.8V, duty cycle
is at maximum ( ~90%).The internal oscillator uses an on-
chip capacitor and trimmed precision current sources to set
the oscillation frequency to 200kHz. Figure 1 shows a 2.5V
output converter. If the Vout <2.5V, then the SENSE voltage
< 0.8V. In this case the error amplifier will be sourcing cur-
rent into the COMP pin so that COMP voltage and duty cycle
will gradually increase.If Vout > 2.5V, the error amplifier will
sink current and reduce the COMP voltage, so that duty cycle
will decrease.The circuit will be in steady state when Vout
=2.5V , Vsense = 0.8V, Icomp = 0. The COMP voltage and
duty cycle depend on Vin.
The under voltage lockout circuit of the SC2608 assures
that both high-side and low-side MOSFET driver outputs
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2005 Semtech Corp.
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oltage Lock
oltage Lock
oltage Lock
oltage Lockout
out
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out
out
The voltage on the COMP
6
remain in the off state whenever the supply voltage drops
below the set threshold. Lockout occurs if V
4.3V typ.
Sof
Sof
Soft Star
Sof
Sof
The SC2608 provides a soft start function to prevent large
inrush currents upon power-up or hiccup retry. If both COMP
and SENSE pins are low (<300mV), the device enters soft
start mode, and the compensation capacitor is slowly charged
by an internal 1.5uA current source. When the COMP pin
reaches 300mV, the low side FET is switched on in order to
refresh the bootstrap capacitor, and begin PWM from a known
state. As the COMP pin rises above 800mV, PWM begins at
minimum duty cycle.
COMP continues to charge, slowly sweeping the device
through the duty cycle range until FB reaches the regulation
point of 800mV. Once FB reaches the regulation point, the
soft start current is switched off, and the strong error amp is
enabled, providing a glitch-free entrance into closed loop
operation. The overcurrent comparator is still active during
soft start mode, and will override soft start in the event that
an overcurrent is
short.
R R R R R
In case of a short circuit or overload, the low-side (LS) FET
will conduct large currents. To protect the regulator in this
situation, the controller will shut down the regulator and be-
gin a soft start cycle later. While the LS driver is on,the Phase
voltage is compared to the OCP trip voltage. If the phase
voltage is lower than OCP trip voltage, an over current condi-
tion is detected. The low-side R
end of each LS-FET turn-on duration. The minimum turn-on
time of the LS-FET is set to be 400nS. This will ensure the
sampled signal is noise free by giving enough time for the
switching noise to die down.
OCP Hiccup
OCP Hiccup
OCP Hiccup
OCP Hiccup
OCP Hiccup
In the event that an overcurrent is detected, the SC2608
latches the fault and begins a hiccup cycle. Switching is
immediately stopped, and the drivers are set to a tristate
condition (Both DH and DL are low). COMP is slowly
discharged to 300mV with an internal 1.5uA current source,
providing a long cooldown time to keep power dissipation
low in the event of a continuous dead short. Once COMP
and SENSE both fall below the 300mV threshold, the part
re-enables the 1.5uA soft start current , and the device begins
a normal startup cycle again.
DS(ON)
DS(ON)
DS(ON)
DS(ON)
DS(ON)
t Star
t Start t t t t
t Star
t Star
Current Limiting
Current Limiting
Current Limiting
Current Limiting
Current Limiting
detected, such as startup into a dead
dson
sense is implemented at
www.semtech.com
SC2608
CC
falls below