UCC1884 Unitrode Semiconductor, UCC1884 Datasheet - Page 4

• Manufacturer Part Number: UCC1884
• Description: Frequency Foldback Current Mode PWM Controller
• Manufacturer: Unitrode Semiconductor

PIN DESCRIPTIONS
CLKSYNC: An edge triggered active low TTL signal to
this pin synchronizes the oscillator to an external clock.
When VOUT decreases below 3.0V, the frequency fold-
back circuit is activated and the controller becomes
unsynchronized. When VOUT exceeds 3.0V, the control-
ler resynchronizes to the external clock.
COMP: The output of the voltage error amplifier used for
compensation. The output is clamped to 3.0V minimum.
CS: Current sense input. This pin accepts a voltage pro-
portional to converter inductor current. The voltage at CS
is compared to the output of the compensated error am-
plifier to control the on-time of the switch. Voltage mode
control can be realized by driving this pin with a fixed
sawtooth ramp. Voltage feedforward is achieved by mak-
ing the peak of this ramp proportional to the input voltage.
CSS: A capacitor, C
time for the power up sequence. This function is also
used when an overcurrent fault occurs. As CSS is
charged, the PWM comparator uses the lowest of either
the voltage at CSS or the error amplifier output voltage to
determine the duty cycle. The duty cycle, therefore,
slowly increases during the soft start cycle. The faults
that cause CSS to discharge and shutdown the controller
are the logical OR of VREF below 4.4V or VDD below
8.8V. If a fault is still present when CSS is discharged be-
low 0.5V, the supply remains off until the fault is cleared.
The soft start time is determined by:
where I
cycle. It does not generate a soft start cycle.
CT: A capacitor, C
creating the oscillator waveform. This waveform varies
between 1.5V and 3.5V. The operating frequency is de-
termined by
The ratio of the time duration of the positive sloped por-
tion of the CT voltage waveform to the period gives the
maximum duty cycle.
FB: The inverting input of the voltage amplifier used to
sense the output voltage. The non-inverting input of the
error amplifier is internally connected to 2.5V.
T
f
SS
C
SS
T
3.5
is 20 A. A current limit terminates the present
R
1.5
4.4
ON
C
I
SS
SS
T
to ground, is charged and discharged
R
SS
3.5
OFF
, to ground programs the soft start
4
GND: The ground pin internally used for all the amplifiers
and as the return for all resistor and capacitor connec-
tions to the UCC3884.
GT: Used to drive an external depletion-mode MOSFET
for the housekeeping power supply. The MOSFET is
turned off when the bootstrap winding voltage exceeds
10V. There is 300mV of hysteresis around the 10V turn-
off voltage to prevent oscillation. See Typical Application.
IOFF: A resistor, R
current of the timing capacitor C
charge current which determines the negative slope of
the oscillator voltage waveform at CT. The discharge
time is dependent on the voltage at the VOUT pin. The
discharge current is given by I
VOUT pin is internally clamped to 3.5V maximum.
ION: A resistor, R
rent of the timing capacitor, C
positive slope of the oscillator waveform. The charge time
is constant and corresponds to the maximum output on-
time at OUT. The charge current equation is I
1.5V/R
C
compensation into the CS pin.
OUT: The output of the controller. The peak source cur-
rent is 0.5A and the peak sink current is 1.0A. The faults
listed under the CSS description turn off this output.
PGND: The power ground pin is used as the return for
the output transistor drive stage.
VDD: The input voltage of the chip. A low ESR and ESL
ceramic capacitor from this pin to GND should be used to
bypass internal switching transients.
VOUT: This pin accomplishes frequency foldback by con-
trolling the discharge current for the oscillator C
capacitor. A dc voltage proportional to the output voltage
is connected to this pin. To startup with zero output volt-
age the user should tie a resistor between VREF and
VOUT. The value depends on the lowest desired operat-
ing frequency. When VOUT decreases below 3.5V the
frequency decreases by reducing the discharge current
I
increasing the discharge current. The maximum operat-
ing frequency occurs when VOUT = 3.5V. The C
time is constant to guarantee a maximum output duty cy-
cle. This pin must be above 3.0V to allow synchronization
to occur.
OFF
T
voltage could be buffered and used to provide slope
. When VOUT increases, the frequency increases by
ON
. When required the linear positive slope of the
ON,
OFF,
to ground programs the charge cur-
to ground, programs the discharge
OFF
T
T
, which generates the
. This is a variable dis-
= VOUT/R
UCC1884
UCC2884
UCC3884
OFF
T
charge
. The
ON
=
T