ISL6226 Datasheet, PDF (12/17 Pages) Intersil Corporation – Advanced PWM and Linear Power Controller for Portable Applications

ISL6226
Description Of Operation
The ISL6226 is a single channel PWM controller and linear
regulator controller intended for chipset, DRAM, audio or
other low voltage power needs of modern notebook and sub-
notebook PCs. The IC integrates control circuits for a single
synchronous buck converter whose output voltage is set in
the range of 0.9V to 0.93*Vin by an external resistive divider
and a linear regulator controller which drives an external
NMOSFET or NPN pass transistor whose output is set in the
range of 0.8V to 3.8V by an external resistor divider. The
synchronous buck converter can be operated in several
modes to optimize the application. The 300kHz frequency
operation is recommended when powered from the battery,
which voltage varies from 5.6V to 24V. In this case the V_IN
pin is connected to the battery terminal and the ramp
generator gain is dynamically adjusted according to the input
voltage.
The IC also can be optimized for a two stage power
conversion configuration where the output voltage is derived
from a 5V input in lieu of the raw battery. Two operating
frequencies of 300kHz or 600kHz are available in this mode.
If 300kHz operation is desired while operating from the 5V
system bus, the V_IN pin should be connected to ground via a
150K ohm resistor. The ramp generator gain is corrected for
the lower input voltage.
When powered from the system 5V rail, the converter can be
configured to operate on increased frequency of 600kHz to
minimize the output filter components. In this case, the V_IN
pin is connected to the ground. Table 1 summarizes the
various modes of operation. Figure 7 below shows plots of
the ramp speed compensation.
Table 1. Configuration for Modes of Operation
Operation
Pin 1
Pin 1 potential
connection
One stage 300kHz
Two stage 300kHz
V_IN
V1>5.6V
150K-GND 1V<V1<2V
Two stage 600kHz
GND V1<0.5V
300kHz CLOCK
600kHz CLOCK
V_-----i_--n_{-- -}t_--
2T
V_-----i_--n_{-- -}t_--
4T
V_-----i_--n_{- -}t_--
8T
Figure 7. Ramp Speed Compensation Vo=2.5V
Current Sensing and Current Limit Protection
The PWM converter uses the lower MOSFET on-state
resistance, r_DS(ON), as the current-sensing element. This
technique eliminates the need for a current sense resistor
and the associated power losses. If more accurate current
protection is desired, current sense resistors may be used in
series with the lower MOSFETs’ source.
A current proportional signal is used to provide average
current mode control and over-current protection. The gain
in the current sense circuit is set by the resistor connected
from ISEN pin to the switching node of buck converter. The
value of this resistor can be estimated by the following
expression:
Risen
=
-^I--^o---^m------^a----^x----^•-----^R----^d----^s----^o---ⁿ--
75 µ A
–
100
where Iomax is the maximum inductor current. The value of
Risen should be specified for the expected maximum
MOSFET operating temperature.
The resulting current out of the ISEN pin through Risen, is
used for current feedback and current limit protection. An
over-current protection threshold is set by an external
resistor connected from OCSET (pin 4) to ground. The value
of this resistor can be obtained from the following
expression:
Rocset = ⁹----^.-⁶-----_I^•--_o---⁽-_c-^R---_•--ⁱ--^sR---^e--_d--ⁿ--_s---–_o----_n1----⁰---⁰----⁾
where Ioc is the value of over current. This is compared
with an internal current limit threshold. When a sampled
value of the output current is determined to be above the
current limit threshold, the PWM drive is terminated and a
counter is initiated. This limits the inductor current build-up
and essentially switches the converter into current-limit
mode. If an overcurrent is detected between 26µs to 53µs
later, an overcurrent shutdown is initiated. If during the
26µs to 53µs period, an overcurrent is not detected, the
counter is reset and sampling continues as normal.
This current limit scheme has proven to be very robust in
applications like portable computers where fast inductor
current build-up is common due to a large difference
between input and output voltages and a low value of the
inductor.
Light-Load (Hysteretic) Operation
In the light-load (hysteretic) mode the output voltage is
regulated by the hysteretic comparator which regulates the
output voltage by maintaining the output voltage ripple as
shown in Figure 8. In Hysteretic mode, the inductor current
flows only when the output voltage reaches the lower limit of
the hysteretic comparator and turns off at the upper limit.
Hysteretic mode saves converter energy at light loads by
supplying energy only at the time when the output voltage
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