ISL6232 Datasheet, PDF (19/25 Pages) Intersil Corporation – High Efficiency System Power Supply Controller for Notebook Computers

ISL6232
TABLE 3. SUMMARY FOR VARIOUS OPERATION MODES
MODE
CONDITION
COMMENT
Shutdown SHDN# = Low.
All circuitry off.
Standby SHDN# = High.
EN3 = EN5 = Low.
LDO5, LDO3, and 2V
reference active. LGATE
stays high.
Soft-Start LDO5>UVLO EN3
or/and EN5 enabled.
Output voltage ramps up
in 1.2ms.
Normal SHDN# = High. EN3 and All circuitry is running.
Operation EN5 enabled.
Discharge
Either output is still high Discharging the output
in standby mode.
through an internal 20Ω
switch from PHASE to
PGND. One output may
still operate while the
other is in discharge
mode. LDO5 active.
Undervoltage Either output is below Lower side MOSFET is
Protection 75% of nominal after a latched on after discharge
20ms blanking time and mode terminates. LDO5 is
output enabled.
active. Reset by toggling
EN3, EN5, SHDN#, VIN
POR.
Overvoltage Either output voltage is Low side MOSFET is
Protection 13% higher than the forced high and high side
nominal.
MOSFET is forced low.
LINEAR REGULATORS AND 2V REFERENCE
In ISL6232, there are two internal regulators available, which
are LDO5 (5V) and LDO3 (3.3V). Once LDO5 is higher than
4.3V, it provides power for buck controllers, 2V reference,
and all the other blocks powered by VCC. The maximum
guaranteed output current that both LDO5 and LDO3
regulators can supply is 100mA. The real maximum current
drawn from the LDOs is determined by the maximum power
dissipation allowed in the package. A short-circuit or
overcurrent limit protection, 170mA (typ), is implemented for
both LDO5 and LDO3. Bypass LDO5 and LDO3 with a 4.7µF
ceramic capacitor.
When OUT5 is larger than the LDO5 switch-over threshold
(4.78V) and after soft-start is finished, LDO5 is shorted to
OUT5 through an internal 2Ω switch and the LDO5 regulator
is disabled to reduce the power dissipation. Similarly, when
OUT3 is larger than the LDO3 switch-over threshold (3.0V)
and after soft-start is finished, LDO3 is shorted to OUT3
through an internal 2.5Ω switch and LDO3 is turned off. All
the internal blocks (powered by VCC) get the power from the
high-efficiency switching power supply instead of the linear
regulator.
The reference voltage, REF, is 2V with a ±1.5% accuracy.
REF provides the reference voltage, 0.8V, for buck
controllers. REF is bypassed to GND with a 0.22µF
capacitor.
Application Information
This section describes how to select the external
components including the inductor, input and output
capacitors, switching MOSFETs, current sensing resistors
and loop compensator design.
The inductor selection has to accommodate trade-offs
between cost, size and efficiency. For example, the lower the
inductance, the smaller the inductor size, but ripple current is
higher; this results in higher AC losses in the magnetic core
and the windings, which decrease the system efficiency. On
the other hand, the higher inductance results in lower ripple
current and smaller output filter capacitors, but higher DCR
(DC resistance of the inductor) loss and slower transient
response. Practical inductor design is based on the inductor
ripple current being ±(15 to 20)% of the maximum operating
DC current at maximum input voltage. The required
inductance can be calculated from Equation 5:
L
=
V_{-----I--N------–Δ----IV--L-O-----U-----T---}
--^V---^O-----^U-----^T--
VIN fs
(EQ. 5)
where V_IN is input voltage, V_OUT is the output voltage, ΔI_L is
the inductor ripple current and f_s is the switching frequency.
The practical inductor ripple current is chosen at 30% of the
output current: ΔI_L= 30% ⋅ I_OUT
For V_IN = 12V, V_OUT = 5V, I_OUT = 5A, and f_s = 300kHz,
L
=
0^-1---.^-2-3^----–×^----5-5^--
------------------⁵--------------------
12 × 300 × 10³
=
6.5 μ H
(EQ. 6)
Ferrite core inductors are often the best choice since they
are optimized at 300kHz to 600kHz operation with low core
loss. The inductor must be large enough not to saturate at
the overcurrent limit I_OC
IOC
=
-⁹---⁵-----^m-----^V--
RCS
(EQ. 7)
One important factor is that the smaller the inductance, the
faster the transient response. One of the parameters limiting
the converters response to load transient is the time required
to change the inductor current. Given a sufficiently fast
control loop design, the ISL6232 can provide either
approximately 5% or 95% duty cycle in response to a load
transient. The response time is the time required to slew the
inductor current from an initial current value to the transient
current level. During this interval the difference between the
inductor current and the transient current level must be
supplied by the output capacitor. Minimizing the response
time can minimize the output capacitance required. The
response time to a transient is different for the application of
load and the removal of load.
19
FN9116.1
April 20, 2009