ISL6443 Datasheet, PDF (13/18 Pages) Intersil Corporation – 300kHz Dual, 180 Degree Out-of-Phase, Step-Down PWM and Single Linear Controller

ISL6443
desired place a current sense resistor in series with the
lower MOSFET source.
Over-Temperature Protection
The IC incorporates an over-temperature protection circuit
that shuts the IC down when a die temperature of 150°C is
reached. Normal operation resumes when the die
temperatures drops below 130°C through the initiation of a
full soft-start cycle.
Implementing Synchronization
The SYNC pin may be used to synchronize two or more
controllers. When the SYNC pins of two controllers are
connected together, one controller becomes the master and
the other controller synchronizes to the master. A pull-down
resistor is required and must be sized to provide a low
enough time constant to pass the SYNC pulse. Connect this
pin to VCC_5V if not used. Figure 16 shows the SYNC pin
waveform operating at 16 times the switching frequency.
following expression estimates the required value of the
current sense resistor depending on the maximum operating
load current and the value of the MOSFET’s r_DS(ON).
R_C
S
≥
-⁽--^I--^M-----^A----^X---⁾---⁽--^R-----^D----^S----⁽--^O-----^N----⁾--⁾
32 µ A
Choosing R_CS to provide 32µA of current to the current
sample and hold circuitry is recommended but values down
to 2µA and up to 100µA can be used.
Due to the current loop feedback, the modulator has a single
pole response with -20dB slope at a frequency determined
by the load.
F_PO
=
----------------¹----------------
2π ⋅ R_O ⋅ C_O
where R_O is load resistance and C_O is load capacitance. For
this type of modulator, a Type 2 compensation circuit is
usually sufficient.
Figure 17 shows a Type 2 amplifier and its response along
with the responses of the current mode modulator and the
converter. The Type 2 amplifier, in addition to the pole at
origin, has a zero-pole pair that causes a flat gain region at
frequencies in between the zero and the pole.
F_Z
=
--------------¹----------------
2π ⋅ R₂ ⋅ C₁
=
6kHz
F_P
=
--------------¹----------------
2π ⋅ R₁ ⋅ C₂
=
600 k H z
FIGURE 16. SYNC WAVEFORM
Feedback Loop Compensation
To reduce the number of external components and to
simplify the process of determining compensation
components, both PWM controllers have internally
compensated error amplifiers. To make internal
compensation possible several design measures were
taken.
First, the ramp signal applied to the PWM comparator is
proportional to the input voltage provided via the VIN pin.
This keeps the modulator gain constant with variation in the
input voltage. Second, the load current proportional signal is
derived from the voltage drop across the lower MOSFET
during the PWM time interval and is subtracted from the
amplified error signal on the comparator input. This creates
an internal current control loop. The resistor connected to
the ISEN pin sets the gain in the current feedback loop. The
13
CONVERTER
EA
G_M = 17.5dB
MODULATOR
F_PO
C2
R2 ^C1
R1
TYPE 2 EA
G_EA = 18dB
F_Z
F_P
F_C
FIGURE 17. FEEDBACK LOOP COMPENSATION
The zero frequency, the amplifier high-frequency gain, and
the modulator gain are chosen to satisfy most typical
applications. The crossover frequency will appear at the
point where the modulator attenuation equals the amplifier
high frequency gain. The only task that the system designer
has to complete is to specify the output filter capacitors to
FN9044.1
October 4, 2005