ISL78020 Datasheet, PDF (10/19 Pages) Intersil Corporation – Automotive Grade TFT-LCD DC/DC with Integrated Amplifiers

ISL78020, ISL78022
FBB
CINT
DRVN
FBN
V_{REF REFERENCE}
GENERATOR
OSCILLATOR
SLOPE COMP OSC
COMPENSATION
GM
AMPLIFIER
Σ
VOLTAGE
AMPLIFIER
PWM
LOGIC
CONTROLLER
BUFFER
UVLO
COMPARATOR
SHUTDOWN
AND START-UP
CONTROL
CURRENT
AMPLIFIER
CURRENT REF
CURRENT
LIMIT COMPARATOR
SS
V_REF
+
-
THERMAL
SHUTDOWN
UVLO
COMPARATOR
BUFFER
SS
+ _0.2V
-
0.4V
UVLO
COMPARATOR
BUFFER
LX
PGND
DRVP
FBP
FIGURE 16. BLOCK DIAGRAM
Boost Converter
The main boost converter is a current mode PWM converter
operating at a fixed frequency. The 1.2MHz switching
frequency enables the use of low profile inductor and
multilayer ceramic capacitors, which results in a compact,
low cost power system for LCD panel design.
The boost converter can operate in continuous or
discontinuous inductor current mode. The ISL78020 and
ISL78022 are designed for continuous current mode, but
they can also operate in discontinuous current mode at light
load. In continuous current mode, current flows continuously
in the inductor during the entire switching cycle in steady
state operation. The voltage conversion ratio in continuous
current mode is given by Equation 1:
V_{-----B----O----O-----S----T-}
V_IN
=
------¹-------
1–D
(EQ. 1)
Where D is the duty cycle of switching MOSFET.
Figure 17 shows the block diagram of the boost controller.
It uses a summing amplifier architecture consisting of GM
stages for voltage feedback, current feedback and slope
compensation. A comparator looks at the peak inductor
current cycle by cycle and terminates the PWM cycle if the
current limit is reached.
An external resistor divider is required to divide the output
voltage down to the nominal reference voltage. Current
drawn by the resistor network should be limited to maintain
the overall converter efficiency. The maximum value of the
resistor network is limited by the feedback input bias current
and the potential for noise being coupled into the feedback
pin. A resistor network in the order of 60kΩ is recommended.
The boost converter output voltage is determined by
Equation 2:
V_BOOST
=
R_{-----1----+-----}R_-----2-
R₁
×
V_REF
(EQ. 2)
10
FN6386.2
December 6, 2007