ISL78020 Datasheet, PDF (12/19 Pages) Intersil Corporation – Automotive Grade TFT-LCD DC/DC with Integrated Amplifiers

ISL78020, ISL78022
Table 2 gives typical values (margins are considered 10%,
3%, 20%, 10% and 15% on V_IN, V_O, L, f_S and I_LMT:
TABLE 2. TYPICAL V_IN, V_O, L, f_S, AND I_OMAX VALUES
V_IN
V_O
L
f_S
I_OMAX
(V)
(V)
(µH)
(MHz)
(mA)
3.3
9
6.8
1.2
898
3.3
12
6.8
1.2
622
3.3
15
6.8
1.2
458
5
9
6.8
1.2
1360
5
12
6.8
1.2
944
5
15
6.8
1.2
694
Input Capacitor
The input capacitor is used to supply the 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 input capacitor should be larger than
maximum input voltage.
Boost Inductor
The boost inductor is a critical part, which influences the
output voltage ripple, transient response, and efficiency.
Value of 3.3µH to 10µH inductor is recommended in
applications to fit the internal slope compensation. The
inductor must be able to handle the following average and
peak current shown in Equations 5 and 6:
I_LPK
=
I_L
A
V
G
+
-^Δ----^I--^L-
2
(EQ. 5)
I_LAVG
=
----^I--^O-------
1–D
(EQ. 6)
Rectifier Diode
A high-speed diode is desired 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.
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 0V.
Compensation
The ISL78020 and ISL78022 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 COMP pin directly to
V_IN will enable P-Mode; for better load regulation, use
PI-Mode with a 2.2nF capacitor and a 180Ω resistor in series
between COMP pin and ground. To improve the transient
response, either the resistor value can be increased or the
capacitor value can be reduced, but too high resistor value
or too low capacitor value will reduce loop stability. Figures 3
through 6 show a comparison of P-Mode vs PI-Mode
performance.
Boost Feedback Resistors
As the boost output voltage, V_BOOST, 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 V_BOOST is reduced, by means of a series
resistor-capacitor network (R₇ and C₇) in parallel with R₁,
with a pole frequency (fp) 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
=
------------------------¹-------------------------
2 × 3.142 × fp × R₇
(EQ. 9)
Linear-Regulator Controllers (V_ON and V_OFF)
The ISL78020 and ISL78022 include 2 independent
linear-regulator controllers, in which there is one positive
output voltage (V_ON), and one negative voltage (V_OFF). The
V_ON and V_OFF linear-regulator controller function diagram,
application circuit and waveforms are shown in Figures 18
and 19 respectively.
12
FN6386.2
December 6, 2007