ISL6237 Datasheet, PDF (31/35 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6237
.
V_IN
UGAUTGEA_2TE2
Q₁
ISL88732
ISL682837733
ISL88734
LGALGTEA_2TE2
Q₂
VOOUUTO_TU2T2
REFRINFE2BF_IN2
R₄
VR
R₃
OUT2
FIGURE 73. SETTING V_OUT2 WITH A VOLTAGE DIVIDER FOR
TRACKING
Inductor Selection
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as follows:
L = -_V----_I^V--_N--^O---_â‹--^U-_f---^T_â‹---_L-^_---_I⁽--_RV-----^I-_â‹-^N--_I--_L-+--_O---^V--_A--^O-_D---^U--₍--_M-^T---^_-_A---⁾_X----₎
(EQ. 7)
Example: I_LOAD(MAX) = 5A, V_IN = 12V, V_OUT2 = 5V,
f = 200kHz, 35% ripple current or LIR = 0.35:
L = ₁----₂----_V-----_â‹-⁵--₂--^V-₀----⁽₀--¹--_k-²--_H--^V---_z---–-_â‹---⁵⁰----_.V-₃----⁾₅-----_â‹---₅---_A--- = 8.3μH
(EQ. 8)
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice. The core must be large enough
not to saturate at the peak inductor current (IPEAK):
IPEAK = I_LOAD(MAX) + [(LIR ⁄ 2) ⋅ I_LOAD(MAX)]
(EQ. 9)
The inductor ripple current also impacts transient response
performance, especially at low V_IN - V_OUT_ differences. Low
inductor values allow the inductor current to slew faster,
replenishing charge removed from the output filter capacitors
by a sudden load step. The peak amplitude of the output
transient (V_SAG) is also a function of the maximum duty
factor, which can be calculated from the on-time and
minimum off-time:
(ΔI_LOAD(MAX))²
⋅
⎛
L⎜
⎝
K
⎛
⎜
⎝
-^V----^O----^U----^T----^_-
V_IN
+
⎞⎞
t
O
FF(M
I
N
)_⎠⎟
⎟
⎠
V_SAG
=
---------------------------------------------------------------------------------------------------------------------------
2 ⋅ C_OUT ⋅ V_OUT
K
⎛
⎜
⎝
-^V----^I--^N-----_V^–----_I^V--_N---^O----^U----^T--_{⎠⎟⎞}
-_tO
F
F
(
M
I
N
)
(EQ. 10)
where minimum off-time = 0.35µs (max) and K is from
Table 2.
Determining the Current 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. The valley of the inductor
current occurs at I_LOAD(MAX) minus half of the ripple
current; therefore:
I_LIMIT(LOW) > I_LOAD(MAX) – [(LIR ⁄ 2) ⋅ I_LOAD(MAX)] (EQ. 11)
where: I_LIMIT(LOW) = minimum current-limit threshold
voltage divided by the r_DS(ON) of Q₂/Q₄.
Use the worst-case maximum value for r_DS(ON) from the
MOSFET Q₂/Q₄ data sheet and add some margin for the
rise in r_DS(ON) with temperature. A good general rule is to
allow 0.2% additional resistance for each °C of temperature
rise.
Examining the 5A circuit example with a maximum
r_DS(ON) = 5mΩ at room temperature. At +125°C reveals the
following:
I_LIMIT(LOW) = (25mV) ⁄ ((5mΩ × 1.2) > 5A – (0.35 ⁄ 2)5A)
(EQ. 12)
4.17A > 4.12A
(EQ. 13)
4.17A is greater than the valley current of 4.12A, so the
circuit can easily deliver the full-rated 5A using the 30mV
nominal current-limit threshold voltage.
Output Capacitor Selection
The output filter capacitor must have low enough equivalent
series resistance (ESR) to meet output ripple and
load-transient requirements, yet have 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's 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:
R_SER
≤
-----------^V----^D----^I--^P------------
I_LOAD(MAX)
(EQ. 14)
where V_DIP is the maximum-tolerable transient voltage drop.
In non-CPU applications, the output capacitor's size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
R_ESR ≤ -_L---_I--_R-----_â‹---_I--_L^V----_OP----_A-^–---_D-^P---₍--_M-----_A----_X----₎
(EQ. 15)
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. Thus, the capacitor is usually
selected by ESR and voltage rating rather than by
capacitance value (this is true of tantalum, OS-CON, and
other electrolytic-type capacitors).
31
FN6418.4
March 18, 2008