ISL78010_11 Datasheet, PDF (11/19 Pages) Intersil Corporation – Automotive Grade TFT-LCD Power Supply

ISL78010
Table 2 gives typical values (margins are considered 10%,
3%, 20%, 10%, and 15%) on V_IN, V_O, L, f_S, and I_OMAX.
TABLE 2. TYPICAL V_IN, V_O, L, f_S, AND I_OMAX VALUES
V_IN (V)
3.3
V_O (V)
9
L (µH)
6.8
f_S
(MHz)
1
I_OMAX
(A)
0.490686
3.3
12
6.8
1
0.307353
3.3
15
6.8
1
0.197353
5
9
6.8
1
0.743464
5
12
6.8
1
0.465686
5
15
6.8
1
0.29902
Input Capacitor
An input capacitor is used to supply the peak charging
current to the converter. It is recommended that C_IN be
larger than 10µF. The reflected ripple voltage will be smaller
with larger C_IN. The voltage rating of the input capacitor
should be larger than the maximum input voltage.
Boost Inductor
The boost inductor is a critical part which influences the
output voltage ripple, transient response, and efficiency.
Values of 3.3µH to 10µH are to match the internal slope
compensation. The inductor must be able to handle the
following average (Equation 5) and peak (Equation 6)
current:
I_LAVG
=
----^I--^O-------
1–D
I_LPK
=
I_L
A
V
G
+
-^Δ----^I--^L-
2
(EQ. 5)
(EQ. 6)
Rectifier Diode
A high-speed diode is necessary due to the high switching
frequency. Schottky diodes are recommended because of
their fast recovery time and low forward voltage. The rectifier
diode must meet the output current and peak inductor
current requirements.
Output Capacitor
The output capacitor supplies the load directly and reduces
the ripple voltage at the output. Output ripple voltage
consists of two components: the voltage drop due to the
inductor ripple current flowing through the ESR of output
capacitor, and the charging and discharging of the output
capacitor (Equation 7).
V_RIPPLE
=
I_LPK
×
ESR
+
-^V----^O-----–-----^V----^I--^N--
V_O
×
------^I--^O-------
C_OUT
×
--¹--
f_S
(EQ. 7)
For low ESR ceramic capacitors, the output ripple is
dominated by the charging and discharging of the output
capacitor. The voltage rating of the output capacitor should
be greater than the maximum output voltage.
NOTE: Capacitors have a voltage coefficient that makes their
effective capacitance drop as the voltage across them increases.
C_OUT in Equation 7 assumes the effective value of the capacitor at a
particular voltage and not the manufacturer’s stated value, measured
at zero volts.
Compensation
The ISL78010 can operate in either P-mode or PI-mode.
P-mode may be preferred in applications where excellent
transient load performance is required but regulation is not
critical. Connecting the CINT pin directly to V_IN will enable
P-mode; for better load regulation, use PI-mode with a 4.7nF
capacitor in series with a 10k resistor between CINT and
ground. This value may be reduced to improve transient
performance; however, very low values will reduce loop
stability. Figures 5 through 10 show a comparison of P-mode
vs PI-mode performance.
Boost Feedback Resistors
As the boost output voltage, A_VDD, is reduced below 12V,
the effective voltage feedback in the IC increases the ratio of
voltage to current feedback at the summing comparator
because R₂ decreases relative to R₁. To maintain stable
operation over the complete current range of the IC, the
voltage feedback to the FBB pin should be reduced
proportionally, as A_VDD is reduced. This can be
accomplished by means of a series resistor-capacitor
network (R₇ and C_7; Equations 8 and 9) in parallel with R₁,
with a pole frequency (f_p) set to approximately 10kHz for
C₂ (effective) = 10µF and 4kHz for C₂ (effective) = 30µF.
R₇
=
⎛⎛
⎝⎝
-₀---_.--₁----¹_×-----_R-----₂-_⎠⎞
– _R---¹--₁-_⎠⎞ ^–1
(EQ. 8)
C₇ = ₂-----_×-----₃---_.--₁----₄---₂--¹---_×-----_f-_p-----_×-----_R----₇-
(EQ. 9)
PI-Mode C_INT (C₂₃) and R_INT (R₁₀)
The IC is designed to operate with a minimum C₂₃ capacitor
of 4.7nF and a minimum C₂ (effective) = 10µF.
Note that, for high voltage A_VDD, the voltage coefficient of
ceramic capacitors (C₂) reduces their effective capacitance
greatly; a 16V, 10µF ceramic can drop to around 3µF at 15V.
To improve the transient load response of A_VDD in PI-mode,
a resistor may be added in series with the C₂₃ capacitor. The
larger the resistor, the lower the overshoot, but at the
expense of stability of the converter loop, especially at high
currents.
With L = 10µH, A_VDD = 15V, and C₂₃ = 4.7nF, C₂ (effective)
should have a capacitance of greater than 10µF. R_INT (R₇)
can have values up to 5kΩ for C₂ (effective) up to 20µF and
up to 10k for C₂ (effective) up to 30µF.
11
FN6501.1
May 3, 2011