ISL6721A_14 Datasheet, PDF (11/24 Pages) Intersil Corporation – Flexible Single-ended Current Mode PWM Controller

ISL6721A
The total supply current (I_C plus I_CC) will be higher,
depending on the load applied to GATE. Total current is the
sum of the quiescent current and the average gate current.
Knowing the operating frequency, f_sw, and the MOSFET
gate charge, Qg, the average GATE output current can be
calculated in Equation 5:
Igate = Qg • f_sw
A
(EQ. 5)
VREF - The 5V reference voltage output. Bypass to LGND
with a 0.01µF or larger capacitor to filter this output as
needed. Using capacitance less than this value may result in
unstable operation.
SS - Connect the soft-start capacitor between this pin and
LGND to control the duration of soft-start. The value of the
capacitor determines both the rate of increase of the duty
cycle during start-up, and also controls the overcurrent
shutdown delay.
ISET - A DC voltage between 0.35V and 1.2V applied to this
input sets the pulse-by-pulse overcurrent threshold. When
overcurrent inception occurs, the SS capacitor begins to
discharge and starts the overcurrent delayed shutdown
cycle.
Thermal Pad (QFN Package Only) - The thermal pad
located on the bottom of the QFN package is electrically
isolated. It is recommended that it be connected to signal
ground.
Functional Description
Features
The ISL6721A current mode PWMs make an ideal choice for
low-cost flyback and forward topology applications requiring
enhanced control and supervisory capability. With adjustable
overvoltage and undervoltage thresholds, overcurrent
threshold, and hic-cup delay, a highly flexible design with
minimal external components is possible. Other features
include peak current mode control, adjustable soft-start,
slope compensation, adjustable oscillator frequency and a
bi-directional synchronization clock input.
Oscillator
The ISL6721A have a sawtooth oscillator with a
programmable frequency range to 1MHz, which can be
programmed with a resistor and capacitor on the RTCT pin.
(Please refer to Figure 4 for the resistance and capacitance
required for a given frequency.)
Implementing Synchronization
The oscillator can be synchronized to an external clock
applied at the SYNC pin or by connecting the SYNC pins of
multiple ICs together. If an external master clock signal is
used, it must be at least 65% of the free running frequency of
the oscillator for proper synchronization. The external
master clock signal should have a pulse width greater than
20ns. If no master clock is used, the first device to assert
SYNC assumes control of the SYNC signal. An external
SYNC pulse is ignored if it occurs during the first 1/3 of the
switching cycle.
During normal operation the RTCT voltage charges from
1.5V to 3.0V and back during each cycle. Clock and SYNC
signals are generated when the 3.0V threshold is reached. If
an external clock signal is detected during the latter 2/3 of
the charging cycle, the oscillator switches to external
synchronization mode and relies upon the external SYNC
signal to terminate the oscillator cycle. The generation of a
SYNC signal is inhibited in this mode. If the RTCT voltage
exceeds 4.0V (i.e. no external SYNC signal terminates the
cycle), the oscillator reverts to the internal clock mode and a
SYNC signal is generated.
Soft-Start Operation
The ISL6721A features soft-start using an external capacitor
in conjunction with an internal current source. Soft-start is
used to reduce voltage stresses and surge currents during
start up.
Upon start up, the soft-start circuitry clamps the error amplifier
output (COMP pin) to a value proportional to the soft-start
voltage. The error amplifier output rises as the soft-start
capacitor voltage rises. This has the effect of increasing the
output pulse width from zero to the steady state operating duty
cycle during the soft-start period. When the soft-start voltage
exceeds the error amplifier voltage, soft-start is completed.
Soft-start forces a controlled output voltage rise. Soft-start
occurs during start-up and after recovery from a fault condition
or overcurrent shutdown. The soft-start voltage is clamped to
4.5V.
Gate Drive
The ISL6721A is capable of sourcing and sinking 1A peak
current. Separate collector supply (V_C) and power ground
(PGnd) pins help isolate the IC’s analog circuitry from the
high power gate drive noise. To limit the peak current
through the IC, an external resistor may be placed between
the totem-pole output of the IC (GATE pin) and the gate of
the MOSFET. This small series resistor also damps any
oscillations caused by the resonant tank of the parasitic
inductances in the traces of the board and the FET’s input
capacitance.
Slope Compensation
For applications where the maximum duty cycle is less than
50%, slope compensation may be used to improve noise
immunity, particularly at lighter loads. The amount of slope
compensation required for noise immunity is determined
empirically, but is generally about 10% of the full scale
current feedback signal. For applications where the duty
cycle is greater than 50%, slope compensation is required to
prevent instability. Slope compensation is a technique in
which the current feedback signal is modified by adding
additional slope to it. The minimum amount of slope
compensation required corresponds to 1/2 the inductor
11
FN6797.0
August 23, 2011