ISL6567_07 Datasheet, PDF (11/26 Pages) Intersil Corporation – Multipurpose Two-Phase Buck PWM Controller with Integrated MOSFET Drivers

ISL6567
the MOSFET on, the lower MOSFET drive turns the
freewheeling element off. The upper MOSFET is kept on
until the clock signals the beginning of the next switching
cycle and the PWM pulse is terminated.
CURRENT SENSING
ISL6567 senses current by sampling the voltage across the
lower MOSFET during its conduction interval. MOSFET
r_DS(ON) sensing is a no-added-cost method to sense current
for load line regulation, channel current balance, module
current sharing, and overcurrent protection.
The ISEN pins are used as current inputs for each channel.
Internally, a virtual ground is created at the ISEN pins. The
R_ISEN resistors are used to size the current flow through the
ISEN pins, proportional to the lower MOSFETs’ r_DS(ON)
voltage, during their conduction periods. The current thus
developed through the ISEN pins is internally averaged, then
the current error signals resulting from comparing the
average to the individual current signals are used for
channel current balancing.
Select the value for the R_ISEN resistors based on the room
temperature r_DS(ON) of the lower MOSFETs and the full-load
total converter output current, I_FL. As this current sense path
R_ISEN
=
5^-r--D-0^---S--×^--(-1^-O--0^{---N-–--6-)-}
⋅
-^I-^F----^L-
2
is also used for OC detection, ensure that at maximum
power train temperature rise and maximum output current
loading the OC protection is not inadvertently tripped. OC
protection current level through the ISEN pins is listed in the
Electrical Specifications table.
CHANNEL-CURRENT BALANCE
Another benefit of multi-phase operation is the thermal
advantage gained by distributing the dissipated heat over
multiple devices and greater area. By doing this, the
designer avoids the complexity of driving multiple parallel
MOSFETs and the expense of using expensive heat sinks
and exotic magnetic materials.
All things being equal, in order to fully realize the thermal
advantage, it is important that each channel in a multi-phase
converter be controlled to deliver about the same current at
any load level. Intersil’s ISL6567 ensure current balance by
comparing each channel’s current to the average current
delivered by both channels and making appropriate
adjustments to each channel’s pulse width based on the
resultant error. The error signal modifies the pulse width to
correct any unbalance and force the error toward zero.
Conversely, should a channel-to-channel imbalance be
desired, such imbalance can be created by adjusting the
individual channel’s R_ISEN resistor. Asymmetrical layouts,
where one phase of the converter is naturally carrying more
current than the other, or where one of the two phases is
subject to a more stringent thermal environment limiting its
current-carrying capability, are instances where this
adjustment is particularly useful, helping to cancel out the
design-intrinsic thermal or current imbalances.
SOFT-START
The soft-start function allows the converter to bring up the
output voltage in a controlled fashion, resulting in a linear
ramp-up. As soon as the controller is fully enabled for
operation, the SS pin starts to output a small current which
charges the external capacitor, C_SS, connected to this pin.
An internal reference clamp controlled by the potential at the
SS pin releases the reference to the input of the error
amplifier with a 1:1 correspondence for SS potential
exceeding 0.7V (typically). Figure 5 details a normal
soft-start startup. The following equation helps determine the
approximate time period during which the controlled output
voltage is ramped from 0V to the desired DC-set level.
t_SS
=
C_{-----S----S-----}⋅_---V_{----R----E----F--}
I_SS
V_OUT (0.5V/DIV)
V_intREF (0.5V/DIV)
GND>
GND>
V_SS (1V/DIV)
EN (5V/DIV)
FIGURE 5. NORMAL SOFT-START WAVEFORMS FOR
ISL6567-BASED MULTI-PHASE CONVERTER
Whenever the ISL6567’s power-on reset falling threshold is
tripped, or it is disabled via the EN pin, the SS capacitor is
quickly discharged via an internal pull-down device
(represented as the 1mA, typical, current source).
As the SS pin’s positive excursion is internally clamped to
about 3.5V, insure that any external pull-up device does not
force more than 3mA into this pin.
Should OC protection be tripped while the ISL6567 is
operating in internal-reference mode and the SS pin not be
allowed to fully discharge the SS capacitor, the ISL6567
cannot continue the normal SS cycling.
11
FN9243.2
March 20, 2007