ISL6540A Datasheet, PDF (14/22 Pages) Intersil Corporation – Single-Phase Buck PWM Controller with Integrated High Speed MOSFET Driver and Pre-Biased Load Capability

ISL6540A
internal linear dropout is too large for a given application.
When using the external linear regulator option, the
LIN_DRV pin should be connected to the gate of a PMOS
device, and a resistor should be connected between its gate
and source. A resistor and a capacitor should be connected
from gate-to-source to compensate the control loop. A PNP
device can be used instead of a PMOS device in which case
the LIN_DRV pin should be connected to the base of the
PNP pass element. The sinking capability of the LIN_DRV
pin is 5mA, and should not be exceeded if using an external
resistor for a PMOS device. The designer should take care
in designing a stable system when using external pass
elements. The VCC pin should be connected to the PVCC
pin with an RC filter to prevent high frequency driver
switching noise from entering the analog circuitry.
High Speed MOSFET Gate Driver
The integrated driver has similar drive capability and
features to Intersil's ISL6605 stand alone gate driver. The
PWM tri-state feature helps prevent a negative transient on
the output voltage when the output is being shut down. This
eliminates the Schottky diode that is used in some systems
for protecting the microprocessor from
reversed-output-voltage damage. See the ISL6605
datasheet for specification parameters that are not defined in
the current ISL6540A “Electrical Specifications” table on
page 6.
A 1Ω to 2Ω resistor is recommended to be in series with the
bootstrap diode when using VCCs above 5.0V to prevent the
bootstrap capacitor from overcharging due to the negative
swing of the trailing edge of the phase node.
Margining Control
When the MAR_CTRL is pulled high or low, the positive or
negative margining functionality is respectively enabled.
When MAR_CTRL is left floating, the function is disabled.
Upon UP margining, an internal buffer drives the OFS- pin
from VCC to maintain OFS+ at 0.591V. The resistor divider,
R_MARG and R_OFS+, causes the voltage at OFS- to be
increased. Similarly, upon DOWN margining, an internal
buffer drives the OFS+ pin from VCC to maintain OFS- at
0.591V. The resistor divider, R_MARG and R_OFS-, causes the
voltage at OFS+ to be increased. In both modes, the voltage
difference between OFS+ and OFS- is then sensed with an
instrumentation amplifier and is converted to the desired
margining voltage by a 5:1 ratio. The maximum designed
margining range of the ISL6540A is ±200mV, this sets the
MINIMUM value of R_OFS+ or R_OFS- at approximately 5.9k
for an R_MARG of 10k for a MAXIMUM of 1V across R_MARG.
The OFS pins are completely independent and can be set to
different margining levels. The maximum usable reference
voltage for the ISL6540A is VCC-1.8V, and should not be
exceeded when using the margining functionality, for
example: V_{REF_MARG} < VCC - 1.8V, as shown in
Equation 8:
V _{M A R G _UP}
=
V_{-----R----E----F-}
5
•
-^R----^M-----^A----^R----^G--
R_OFS+
V _{M A R G _DOWN}
=
V_{-----R----E----F-}
5
•
-^R----^M-----^A----^R----^G--
R_OFS-
(EQ. 8)
An alternative calculation provides for a desired percentage
change in the output voltage when using the internal 0.591V
reference:
V _{P C T _UP}
=
20 • ^R-----^M-----^A----^R----^G--
R_OFS+
V _{p c t _DOWN}
=
20
•
R_{-----M-----A----R----G--}
R_OFS-
(EQ. 9)
When not used in a design OFS+, OFS-, and MARCTRL
should be left floating. To prevent damage to the part, OFS+
and OFS- should not be tied to VCC or PVCC.
Reference Output Buffer
The internal buffer’s output tracks the unmargined system
reference. It has a 19mA drive capability, with maximum and
minimum output voltage capabilities of VCC and GND
respectively. Its capacitive loading can range from 1µF to
above 17.6µF, which is designed for 1 to 8 DIMM systems in
DDR (Dual Data Rate) applications. 1µF of capacitance
should always be present on REFOUT. It is not designed to
drive a resistive load and any such load added to the system
should be kept above 300kΩ total impedance. The
Reference Output Buffer should not be left floating.
Reference Input
The REFIN pin allows the user to bypass the internal 0.591V
reference with an external reference. Asynchronously, if
REFIN is NOT within ~1.8V of VCC, the external reference
pin is used as the control reference instead of the internal
0.591V reference. The minimum usable REFIN voltage is
~68mV, while the maximum is VCC - 1.8V - V_MARG (if
present).
REFERENCE
V_REF = 0.591V
VCC
ISL6540A
STATE
MACHINE
REFIN
800mV
REFOUT
MARGINING ^VREF_MARG
BLOCK
OTA
FIGURE 5. SIMPLIFIED REFERENCE BUFFER
14
FN6288.5
October 7, 2008