ISL78419 Datasheet, PDF (11/20 Pages) Intersil Corporation – Integrated Automotive TFT-LCD Power Supply Regulator

ISL78419
Applications Information
Enable Control
With VIN > UVLO, only the Logic output channel is activated. All
other functions in ISL78419 are shut down when the enable pin
is pulled down. When the voltage at the enable pin reaches high
threshold, the whole chip turns on.
Frequency Selection
The ISL78419 switching frequency can be user selected to
operate at either constant 600kHz or 1.2MHz. Lower switching
frequency can save power dissipation at very light load
conditions. Also, low switching frequency more easily leads to
discontinuous conduction mode, while higher switching
frequency allows for smaller external components, such as
inductor and output capacitors, etc. Higher switching frequency
will get higher efficiency within some loading ranges depending
on VIN, VOUT, and external components, as shown in Figure 1.
Connecting the FREQ pin to GND sets the PWM switching
frequency to 600kHz, or connecting FREQ pin to V_IN for 1.2MHz.
Soft-Start
The soft-start is provided by an internal current source to charge
the external soft-start capacitor. The ISL78419 ramps up the
current limit from 0A up to the full value, as the voltage at the SS
pin ramps from 0V to 0.8V. Hence, the soft-start time is 3.2ms
when the soft-start capacitor is 22nF, 6.8ms for 47nF and
14.5ms for 100nF.
Operation
The boost converter is a current mode PWM converter operating
at either 600kHz or 1.2MHz. It can operate in both discontinuous
conduction mode (DCM) at light load and continuous conduction
mode (CCM). In continuous conduction 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 the switching MOSFET.
The boost regulator 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-
R₂
×
V_F
B
(EQ. 2)
The current through the MOSFET is limited to 1.5A_PEAK.
This restricts the maximum output current (average) based on
Equation 3:
I_OMAX
=
⎛
⎝
I_L
MT
–
-^Δ--₂--^I--^L-_⎠⎞
×
V_-----I--N--
V_O
(EQ. 3)
Where ΔI_L is the peak-to-peak inductor ripple current, and is set
by Equation 4:
ΔI_L
=
-^V----^I--^N--
L
×
-^D--
f_s
(EQ. 4)
Where f_S is the switching frequency (600kHz or 1.2MHz).
Capacitor
An input capacitor is used to suppress the voltage ripple injected
into the boost converter. The ceramic capacitor with a
capacitance larger than 10µF is recommended. The voltage
rating of the input capacitor should be larger than the maximum
input voltage. Some input capacitors are recommended in Table 1.
TABLE 1. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATION
CAPACITOR
SIZE MFG
PART NUMBER
10µF/6.3V
0603 TDK
C1608X5R0J106M
10µF/16V
1206 TDK
C3216X7R1C106M
10µF/10V
0805 Murata
GRM21BR61A106K
22µF/10V
1210 Murata
GRB32ER61A226K
Inductor
The boost inductor is a critical part that influences the output
voltage ripple, transient response, and efficiency. Values of
3.3µH to 10µH are used to match the internal slope
compensation. The inductor must be able to handle the following
average and peak currents shown in Equation 5:
I_LAVG
I_LPK =
=
----^I--^O-------
1–D
I_LAVG
+
-^Δ----^I--^L-
2
(EQ. 5)
Some inductors are recommended in Table 2 for different design
considerations.
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 reverse voltage
rating of this diode should be higher than the maximum output
voltage. The rectifier diode must meet the output current and
peak inductor current requirements. Table 3 shows some
recommendations for boost converter diode.
TABLE 2. BOOST CONVERTER INDUCTOR RECOMMENDATION
DIMENSIONS
INDUCTOR (mm)
MFG
PART
NUMBER
NOTE
10µH/ 8.3x8.3x4.5 Sumida CDR8D43-100NC
4Apeak
Efficiency
optimization
6.8µH/ 5.0x5.0x2.0 TDK PLF5020T-6R8M1R8
1.8Apeak
10µH/ 6.6x7.3x1.2 Cyntec PCME061B-100MS
2.2Apeak
PCB
space/profile
optimization
11
FN8292.1
December 3, 2012