ISL6232 Datasheet, PDF (20/25 Pages) Intersil Corporation – High Efficiency System Power Supply Controller for Notebook Computers

ISL6232
Equation 8 gives the approximate response time interval for
application and removal of a step transient load:
t_r
i
se
≈
V^-----_I^-L-_N^{---I--S-–----TV----EO----P--}_U^----_T^{-- ,}
Tfa
l
l
≈
L_----I_{--S-----T----E----P---}
VOUT
(EQ. 8)
Where I_STEP is the transient load current step, t_rise and t_fall
are the response time to the application and the removal of
load, respectively. The worst-case response time can be
either at the application or removal of load. Be sure to check
both of these equations at the minimum and maximum
output levels for the worst-case response time.
Determining the Overcurrent Limit
The minimum current-limit threshold must be great enough
to support the maximum load current when the current limit
is at the minimum tolerance value. ISL6232 uses peak
current detection. The peak inductor current occurs at
I_OUT,MAX plus half of the ripple current; therefore,
IL
IMIT
>
IOUTMAX
+
Δ_----I_--L--
2
(EQ. 9)
The minimum current-limit threshold voltage is 65mV. For
accurate current sense-resistor with 8mΩ, the current limit
I_LIMIT is 8.1A, which is higher than 5.75A, calculated from
the above equation. So, the circuit can easily deliver
full-rated 5A using 65mV current limit threshold.
For DCR of inductor current sensing (Refer to Figure 29), if
the voltage drop across the DCR of the inductor is higher
than 65mV, then a resistor divider across the inductor has to
be used so that the output voltage across the capacitor
reaches current limit threshold (65mV minimum) at the
maximum DCR. The inductor time constant has to match
with the RC current sensing network for good current
sensing accuracy, that is,
-----^L---¹------
Rdc1
≤
---^R-----⁷----^R-----⁸----
R7 + R8
C16
(EQ. 10)
This requirement is not so stringent because it is used for
overcurrent protection and not for the adaptive output
voltage positioning applications. Besides, DCR of the
inductor is also a function of the temperature. A good
general rule for copper is to allow 3.9% additional resistance
for each 10°C of temperature rise. Since there is 1MΩ input
impedance from CS to ground, to achieve good current
sensing accuracy, R₇, and R₈ have to meet the following
inequality:
-------^V----^O-----^U-----^T-------- ≤ 2mV
1
+
-------^R----^X--------
R7 // R8
(EQ. 11)
Where Rx is the input impedance from CS to ground.
Given RDC1 = 15mΩ at +85°C, L = 6.8µH, we choose
R₇ = 1.5kΩ, R₈ = 3kΩ, and C₁₆ = 0.47µF.
Check the current limit I_LIMIT as shown in Equation 12:
-------^R-----⁸--------
R₇ + R₈
R_dc
1
×
I_L
I
MIT
=
65 m V
(EQ. 12)
We have I_LIMIT = 6.5A. Therefore, the circuit can easily
deliver the fully rated 5A current.
Output Capacitor Selection
The output filter capacitor must have low enough equivalent
series resistance (ESR) to meet output ripple and
load-transient requirements. The ISL6232 uses peak current
mode control, which does not require high enough ESR to
satisfy stability requirements. The output capacitance must
also be high enough to absorb the inductor energy while
transitioning from full-load to no-load conditions without
tripping the overvoltage fault latch. In applications where the
output is subject to large load transients, the output capacitor
size depends on how much ESR is needed to prevent the
output from dipping too low under a load transient, ignoring
the sag due to finite capacitance.
The ESR of the output capacitors has to meet the following
equation:
E
S
R
<
--^V----^D----^I--^P----
I_STEP
(EQ. 13)
where V_DIP is the maximum tolerable transient voltage drop
or rise. In system power applications, the ESR of the output
capacitors usually determines the steady-state output
voltage ripple, which is practically designed below 1% of the
output voltage. Thus, we have
ESR ≤ -^V--_Δ--^P-_I--^-L--^P--
(EQ. 14)
where V_P-P is the peak-to-peak output voltage ripple. The
actual capacitance value required relates to the physical size
needed to achieve low ESR, as well as to the chemistry of
the capacitor technology and loop bandwidth.
Since the voltage dip or spike due to loop transient response
is usually smaller than that of voltage dip or spike due to
ESR during the load step transient, the capacitor is usually
selected by ESR and voltage rating rather than by
capacitance value. The commonly used output capacitors
are POSCAP from Sanyo and SPCAP from Panasonic due
to smaller size, low ESR and reasonable price.
Most power supplies requires an overall voltage accuracy of
±5%, including steady-state tolerance, steady-state output
ripple, line regulation and step load transient tolerance. The
ISL6232 has ±1.5% accuracy for the band gap, ±0.5% for
steady-state output ripple and line regulation. This allows
±3% tolerance due to the step load transient. For 5V output,
the required ESR is given by Equation 15:
ESR ≤
3_----%_-------×_-----5_----V_--
3A
=
50 m Ω
(Assume 3A step load)
(EQ. 15)
20
FN9116.1
April 20, 2009