ISL6740 Datasheet, PDF (12/29 Pages) Intersil Corporation – Flexible Double Ended Voltage and Current Mode PWM Controllers

ISL6740, 1SL6741
Oscillator
The ISL6740, ISL6741 have an oscillator with a
programmable frequency range to 2MHz, which can be
programmed with two resistors and capacitor. The use of
three timing elements, R_TC, R_TD, and C_T allow great
flexibility and precision when setting the oscillator frequency.
The switching period may be considered the sum of the
timing capacitor charge and discharge durations. The charge
duration is determined by R_TC and C_T. The discharge
duration is determined by R_TD and C_T.
T_C ≈ 0.5 • R_TC • C_T
S
(EQ. 2)
T_D ≈ 0.02 • R_TD • C_T
S
T_SW
=
T_C + T_D
=
-----¹-------
F_SW
S
(EQ. 3)
(EQ. 4)
where T_C and T_D are the charge and discharge times,
respectively, T_SW is the oscillator free running period, and f
is the oscillator frequency. One output switching cycle
requires two oscillator cycles. The actual times will be
slightly longer than calculated due to internal propagation
delays of approximately 10ns/transition. This delay ads
directly to the switching duration, but also causes overshoot
of the timing capacitor peak and valley voltage thresholds,
effectively increasing the peak-to-peak voltage on the timing
capacitor. Additionally, if very low charge and discharge
currents are used, there will be increased error due to the
input impedance at the C_T pin.
The maximum duty cycle, D, and percent deadtime, DT, can
be calculated from:
D
=
---^T----^C-----
T_SW
(EQ. 5)
DT = 1 – D
(EQ. 6)
Implementing Synchronization
The oscillator can be synchronized to an external clock
applied to the SYNC pin or by connecting the SYNC pins of
multiple ICs together. If an external master clock signal is
used, the free running frequency of the oscillator should be
~10% slower than the desired synchronous frequency. The
external master clock signal should have a pulse width
greater than 20ns. The SYNC circuitry will not respond to an
external signal during the first 60% of the oscillator switching
cycle.
The SYNC input is edge triggered and its duration does not
affect oscillator operation. However, the deadtime is affected
by the SYNC frequency. A higher frequency signal applied to
the SYNC input will shorten the deadtime. The shortened
deadtime is the result of the timing capacitor charge cycle
being prematurely terminated by the external SYNC pulse.
Consequently, the timing capacitor is not fully charged when
the discharge cycle begins. This effect is only a concern
when an external master clock is used, or if units with
different operating frequencies are paralleled.
Soft-start Operation
The ISL6740, ISL6741 feature a soft-start using an external
capacitor in conjunction with an internal current source. soft-
start reduces stresses and surge currents during start up.
Upon start up, the soft-start circuitry clamps the error voltage
input (V_ERROR pin) indirectly to a value equal to the soft-
start voltage. The soft-start clamp does not actually clamp
the error voltage input as is done in many implementations.
Rather the PWM comparator has two inverting inputs such
that the lower voltage is in control.
The output pulse width increases as the soft-start capacitor
voltage increases. This has the effect of increasing the duty
cycle from zero to the regulation pulse width during the soft-
start period. When the soft-start voltage exceeds the error
voltage, soft-start is completed. soft-start occurs during
start-up, after recovery from a Fault condition or
overcurrent/short circuit shutdown. The soft-start voltage is
clamped to 4.5V.
The Fault signal output is high impedance during the soft-
start cycle. A pull-up resistor to VREF or a pull-down resistor
to ground should be added to achieve the desired state of
Fault during soft-start.
Gate Drive
The ISL6740, ISL6741 are capable of sourcing and sinking
0.5A peak current, but are primarily intended to be used in
conjunction with a MOSFET driver due to the 5V drive level.
To limit the peak current through the IC, an external resistor
may be placed between the totem-pole output of the IC
(OUTA or OUTB 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.
Undervoltage Monitor and Inhibit
The UV input is used for input source undervoltage lockout
and inhibit functions. If the node voltage falls below 1.00V a
UV shutdown fault occurs. This may be caused by low
source voltage or by intentional grounding of the pin to
disable the outputs. There is a nominal 10μA switched
current source used to create hysteresis. The current source
is active only during an UV/Inhibit fault; otherwise, it is
inactive and does not affect the node voltage. The
magnitude of the hysteresis is a function of the external
resistor divider impedance. If the resistor divider impedance
results in too little hysteresis, a series resistor between the
UV pin and the divider may be used to increase the
hysteresis. A soft-start cycle begins when the UV/Inhibit fault
clears.
12
FN9111.4
July 13, 2007