ISL6742B Datasheet, PDF (13/20 Pages) Intersil Corporation – Fast current sense to output delay

ISL6742B
amplifier (IEA). The IEA is similar to the voltage EA found in most
PWM controllers, except it cannot source current. Instead, VERR
has a separate internal 1mA pull-up current source.
Configure the IEA as an integrating (Type I) amplifier using the
internal 0.6V reference. The voltage applied at FB is integrated
against the 0.6V reference. The resulting signal, VERR, is applied
to the PWM comparator where it is compared to the sawtooth
voltage on RAMP. If FB is less than 0.6V, the IEA will be open loop
(can’t source current), VERR will be at a level determined by the
voltage loop, and the duty cycle is unaffected. As the output load
increases, I_OUT will increase, and the voltage applied to FB will
increase until it reaches 0.6V. At this point the IEA will reduce
VERR as required to maintain the output current at the level that
corresponds to the 0.6V reference. When the output current
again drops below the average current limit threshold, the IEA
returns to an open loop condition, and the duty cycle is again
controlled by the voltage loop.
The average current control loop behaves much the same as the
voltage control loop found in typical power supplies except it
regulates current rather than voltage.
The EA available on the ISL6742B may also be used as the
voltage EA for the voltage feedback control loop rather than the
current EA as described previously. An external op amp may be
used as either the current or voltage EA providing the circuit is
not allowed to source current into VERR. The external EA must
only sink current, which may be accomplished by adding a diode
in series with its output.
The 4x gain of the sample and hold buffer allows a range of
150mV to 1000mV peak on the CS signal, depending on the
resistor divider placed on IOUT. The overall bandwidth of the
average current loop is determined by the integrating current EA
compensation and the divider on IOUT.
C₁₀
150mV TO
1000mV
R₆
1
ISL6742B
2 VERR
1
16
3
15
4
14
5 FB -
13
6 ^{0.6V +}
12
7 CS
S&H
4x
8 IOUT
11
90
R₅
The average current loop bandwidth is normally set to be much
less than the switching frequency, typically less than 5kHz and
often as slow as a few hundred hertz or less. This is especially
useful if the application experiences large surges. The average
current loop can be set to the steady state overcurrent threshold
and have a time response that is longer than the required
transient. The peak current limit can be set higher than the
expected transient so that it does not interfere with the transient,
but still protects for short-term larger faults. In essence, a 2-stage
overcurrent response is possible.
The peak overcurrent behavior is similar to most other PWM
controllers. If the peak current exceeds 1V, the active output
pulse is terminated immediately.
If voltage-mode control is used in a bridge topology, it should be
noted that peak current limit results in inherently unstable
operation. DC blocking capacitors used in voltage-mode bridge
topologies become unbalanced, as does the flux in the
transformer core. The average overcurrent circuitry prevents this
behavior by maintaining symmetric duty cycles for each
half-cycle. If the average current limit circuitry is not used, a
latching overcurrent shutdown method using external
components is recommended.
The CS to output propagation delay is increased by the Leading
Edge Blanking (LEB) interval. The effective delay is the sum of
the two delays and is 130ns maximum.
Voltage Feed-Forward Operation
Voltage feed-forward is a technique used to regulate the output
voltage for changes in input voltage without the intervention of
the control loop. Voltage feed-forward is often implemented in
voltage-mode control loops, but is redundant and unnecessary in
peak current-mode control loops.
Voltage feed-forward operates by modulating the sawtooth ramp
in direct proportion to the input voltage. Figure 11 demonstrates
the concept.
VIN
ERROR VOLTAGE
RAMP
CT
R₄
OUTA, OUTB
FIGURE 10. AVERAGE OVERCURRENT IMPLEMENTATION
The current EA crossover frequency, assuming R₆ >> (R₄||R₅), is
expressed in Equation 7:
f_CO = -₂---_-----_---_R----¹-₆----_---_C-----₁---₀--
Hz
(EQ. 7)
Where f_CO is the crossover frequency. A capacitor in parallel with
R₄ may be used to provide a double-pole roll-off.
FIGURE 11. VOLTAGE FEED-FORWARD BEHAVIOR
Input voltage feed-forward may be implemented using the RAMP
input. An RC network connected between the input voltage and
ground, as shown in Figure 12 on page 14, generates a voltage
ramp proportional to the amplitude of the source voltage. At the
termination of the active output pulse, RAMP is discharged to
ground so that a repetitive sawtooth waveform is created. The
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FN8565.1
November 3, 2015