ISL62381 Datasheet, PDF (18/23 Pages) Intersil Corporation – High-Efficiency, Quad or Triple-Output System Power Supply Controller for Notebook Computers

ISL62381, ISL62382, ISL62383
COMP signal. R_TOP, R_FB, C_FB and C_INT form the Type-II
compensator. The frequency domain transfer function is given
by Equation15:
G_COMP(s)
=
-----------¹-----+-----^s----^•--⁽---^R----^T----^O-----^P-----+----^R-----^F----^B----⁾--^•---^C-----^F---^B-------------
s • R_TOP • C_INT • (1 + s • R_FB • C_FB)
(EQ. 15)
C_INT = 100pF
R_FB
C_FB
R_TOP
-
VO
FB
COMP
EA
+
R_BOTTOM
REF
ISL6238
FIGURE 27. COMPENSATION REFERENCE CIRCUIT
The LC output filter has a double pole at its resonant frequency
that causes rapid phase change. The R³ modulator used in the
ISL62381, ISL62382 and ISL62383 make the LC output filter
resemble a first order system in which the closed loop stability
can be achieved with the recommended Type-II compensation
network. Intersil provides a PC-based tool (example page is
shown later) that can be used to calculate compensation
network component values and help simulate the loop
frequency response.
LDO5 Linear Regulator
In addition to the two SMPS outputs, the ISL62381, ISL62382
and ISL62383 also provide two linear regulator outputs. LDO5
is fixed 5V LDO output capable of sourcing 100mA continuous
current.
When the output of SMPS2 is programmed to 5V, SMPS2 will
automatically take over the load of LDO5. This provides a
large power savings and boosts the efficiency. After
switchover to SMPS2, the LDO5 output current plus the
MOSFET drive current should not exceed 100mA in order to
gurarantee the LDO5 output voltage in the range of 5V ±5%.
The total MOSFET drive current can be estimated by
Equation 16.
I_DRIVE = Q_g ⋅ F_SW
(EQ. 16)
where Q_g is the total gate charge of all the power MOSFET
in two SMPS regulators. Then the LDO5 output load current
should be less than 100mA-I_DRIVE.
LDO3 Linear Regulator
ISL62381, ISL62382 includes LDO3 linear regulator whose
output is adjustable from 1.2V to 5V through LDO3FB pin with a
1.2V reference voltage. It can be independently enabled from
both SMPS channels. Logic high of LDO3EN will enable LDO3.
LDO3 is capable of sourcing 100mA continuous current and
draws its power from LDO3IN pin, which must be connected to
a voltage greater than the LDO3 output voltage plus the
dropout voltage.
Currents in excess of the limit will cause the LDO3 voltage to
drop dramatically, limiting the power dissipation.
Thermal Monitor and Protection
LDO3 and LDO5 can dissipate non-trivial power inside the
ISL62381, ISL62382 and ISL62383 at high input-to-output
voltage ratios and full load conditions. To protect the silicon,
ISL62381, ISL62382 and ISL62383 continually monitor the
die temperature. If the temperature exceeds +150°C, all
outputs will be turned off to sharply curtail power dissipation.
The outputs will remain off until the junction temperature has
fallen below +135°C.
General Application Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to design a single-phase
power converter. It is assumed that the reader is familiar with
many of the basic skills and techniques referenced in the
following section. In addition to this guide, Intersil provides
complete reference designs that include schematics, bills of
materials, and example board layouts.
Selecting the LC Output Filter
The duty cycle of an ideal buck converter is a function of the
input and the output voltage. This relationship is written as
Equation 17:
D
=
V_{-----O----U----T--}
V_IN
(EQ. 17)
The output inductor peak-to-peak ripple current is written as
Equation 18:
I_PP
=
V_{-----O----U----T-----}•_--(_---1_{----–-----}D_-----)
F_SW • L
(EQ. 18)
A typical step-down DC/DC converter will have an I_P-P of
20% to 40% of the maximum DC output load current. The
value of I_P-P is selected based upon several criteria such as
MOSFET switching loss, inductor core loss, and the resistive
loss of the inductor winding. The DC copper loss of the
inductor can be estimated by Equation 19:
P_COPPER = I_LOAD2 • DCR
(EQ. 19)
Where I_LOAD is the converter output DC current.
The copper loss can be significant so attention has to be given
to the DCR selection. Another factor to consider when choosing
the inductor is its saturation characteristics at elevated
temperatures. A saturated inductor could cause destruction of
circuit components, as well as nuisance OCP faults.
A DC/DC buck regulator must have output capacitance C_O
into which ripple current I_P-P can flow. Current I_P-P develops
out of the capacitor. These two voltages are written as
Equation 20:
ΔV_ESR = I_PP • ESR
(EQ. 20)
18
FN6665.4
August 7, 2008