SC1104A Datasheet, PDF (6/14 Pages) Semtech Corporation – Simple, Synchronous Voltage Mode PWM Controller

SC1104A/B
POWER MANAGEMENT
Theory of Operation
Synchronous Buck Converter
The output voltage of the synchronous converter is set
and controlled by the output of the error amplifier. The
inverting 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 1V reference.
The circuit will be in steady state when Vout =3.3V ,
Vsense = 1V, Icomp = 0 . The COMP voltage and duty
cycle depend on Vin.
Under Voltage Lockout
The error amplifier output is connected to the
COMPensation pin. The error amplifier generates a cur-
rent proportional to (Vsense – 1V), which is the COMP
pin output current (Transconductance ~ 12mS). The
voltage on the COMP pin is the integral of the error am-
plifier current. The COMP voltage is the non-inverting
input to the PWM comparator and controls the duty cycle
of the MOSFET drivers. The size of capacitor Ccomp con-
trols the stability and transient response of the regula-
tor. The larger the capacitor, the slower the COMP volt-
age changes, and the slower the duty cycle changes.
The inverting input voltage of the PWM comparator is
the triangular output of the oscillator.
When the oscillator output voltage drops below the COMP
voltage, the comparator output goes high. This pulls DL
low, turning off the low-side FET. After a short delay (“dead
time”), DH is pulled high, turning on the high-side FET.
When the oscillator voltage rises back above the error
amplifier output voltage, the comparator output goes low.
This pulls DH low, turning off the high-side FET, and after
a dead time delay, DL is pulled high, turning on the low-
side FET. The dead time delay is determined by a
monostable on the chip.
The triangle wave minimum is about 1V, and the maxi-
mum is about 2V. Thus, if Vcomp = 0.9V, high side duty
cycle is the minimum (~0%) , but if Vcomp is 2.0V, 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 300kHz
(SC1104A) or 600kHz (SC1104B).
Figure 1 shows a 3.3V output converter. If the Vout <3.3V,
then the SENSE voltage < 1V. In this case the error
amplifier will be sourcing current into the COMP pin so
that COMP voltage and duty cycle will gradually increase.
If Vout > 3.3V, the error amplifier will sink current and
reduce the COMP voltage, so that duty cycle will decrease.
The under voltage lockout circuit of the SC1104A/B as-
sures that both high-side and low-side MOSFET driver
outputs remain in the off state whenever the supply volt-
age drops below set parameters. Lockout occurs if V_CC
falls below 4.2V typ.
R_DS(ON) Current Limiting
In case of a short circuit or overload, the high-side (HS)
FET will conduct large currents. To prevent damage, in
this situation, large currents will generate a fault condi-
tion and begin a soft start cycle.
While the HS driver is on, the phase voltage is compared
to the Vcc pin voltage. If the phase voltage is 200mV
lower than Vcc, a fault is latched and the soft start cycle
begins.
The voltages are compared during the middle of the HS
pulse, to prevent transients from affecting the accuracy.
The sampling of the voltage across the top FET occurs
after a time delay t_DELAY = 100ns_typ from the time the
DH is pulled high. This delay prevents the measurement
to be effected by ringing on the leading edge of the phase
node pulse. The duration of the sampling is t_SAMPLE =
100ns_typ. It is being disabled at very low duty cycle when
t_ON < 300ns_typ. This feature allows for the orderly start-
up during the inrush of the current charging output ca-
pacitor and the fault free operation with extremely high
input/output voltage ratio, e.g., V_IN = 12V and V_OUT = 1V.
The over-current comparator (OC) is only active if the
phase node is > 3.3V. This means that in the case of
power source being < 3V the OC will be disabled even
though the rest of the circuitry is completely functional.
SC1104 still can be used for stepping down, e.g. 2.8V to
2.5V, 2V, 1.8V, etc.
© 2006 Semtech Corp.
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