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EL7512C Schematic ( PDF Datasheet ) - Elantec Semiconductor

Teilenummer EL7512C
Beschreibung High Frequency PWM Step-Up Regulator
Hersteller Elantec Semiconductor
Logo Elantec Semiconductor Logo 




Gesamt 9 Seiten
EL7512C Datasheet, Funktion
EL7512C - Preliminary
High Frequency PWM Step-Up Regulator
Features
• 90% efficiency
• Up to 600mA IOUT
• 5V < VOUT < 18V
• VIN > 2V
• Up to 1.2MHz adjustable
frequency
• < 3µA shutdown current
• Adjustable soft-start
• Low battery detection
• Internal thermal protection
• 1.1mm max height 10-pin MSOP
package
Applications
• 3V to 5V, 12V, and 18V
converters
• 5V to 12V and 16V converters
• TFT-LCD
• DSL
• Portable equipment
• Desktop equipment
Ordering Information
Part No
EL7512CY
Package
10-Pin MSOP
Tape & Reel
Outline #
MDP0043
General Description
The EL7512C is a high frequency, high efficiency step-up DC:DC
regulator operated at fixed frequency PWM mode. With an integrated
1A MOSFET, it can deliver up to 600mA output current at up to 90%
efficiency. The adjustable switching frequency is up to 1.2MHz, mak-
ing it ideal for DSL applications.
When shut down, it draws <3µA of current. This feature, along with
the minimum starting voltage of 2V, makes it suitable for portable
equipment powered by one lithium ion or 3 to 4 NiMH cells.
The EL7512C is available in a 10-pin MSOP package, with maximum
height of 1.1mm. With proper external components, the whole con-
verter takes less than 0.25in2 PCB space.
This device is specified for operation over the full -40°C to +85°C
temperature range.
Typical Application Diagram
VIN
(2V-
9V)
10µF
100k
10µH
1 PGND
LX 10
2 SGND
VDD 9
3 RT
FB 8
4 EN
SS 7
5 LBI
LBO 6
EL7512C
VOUT
(12V up
47µF
to
400mA)
20nF
80.6k
10k
4.7nF
Note: All information contained in this data sheet has been carefully checked and is believed to be accurate as of the date of publication; however, this data sheet cannot be a “controlled document”. Current revisions, if any, to these
specifications are maintained at the factory and are available upon your request. We recommend checking the revision level before finalization of your design documentation.
© 2001 Elantec Semiconductor, Inc.






EL7512C Datasheet, Funktion
EL7512C - Preliminary
High Frequency PWM Step-Up Regulator
Applications Information
The EL7512C is a step-up regulator, operated at fixed
frequency pulse-width-modulation (PWM) control. The
input voltage is 2V-12V and output voltage is 5V-16V.
The switching frequency (up to 1.2MHz) is decided by
the resistor connected to RT pin.
Start-up
After VDD reaches a threshold of about than 2V, the
power MOSFET is controlled by the start-up oscillator,
which generates fixed duty-ratio of 0.5-0.7 at a fre-
quency of several hundred kilohertz. This will boost the
output voltage.
When VDD reaches about 3.7V, the PWM comparator
takes over the control. The duty ratio will be decided by
the multiple-input direct summing comparator,
Max_Duty signal (about 90% duty-ratio), and the Cur-
rent Limit Comparator, whichever is the smallest.
The soft-start is provided by the current limit compari-
tor. As the internal 12µA current source changes the
external CSS, the peak MOSFET current is limited by
the voltage on the capacitor. This in turn controls the ris-
ing rate of output voltage.
The regulator goes through the start-up sequence as well
after the EN signal is pulled to HI.
Steady-State Operation
When the output reaches the preset voltage, the regulator
operates at steady state. Depending on the input/output
condition and component, the inductor operates at either
continuous-conduction mode or discontinuous-conduc-
tion mode.
In the continuous-conduction mode, the inductor current
is a triangular waveform and LX voltage a pulse wave-
form. In the discontinuous-conduction mode, the
inductor current is complete dry out before the MOSFET
is turned on again. The input voltage source, the induc-
tor, and the MOSFET and output diode parasitic
capacitors forms a resonant circuit. Oscillation will
occur in this period. This oscillation is normal and will
not affect the regulation.
At very low load, the MOSFET will skip pulse some-
times. This is normal.
Current Limit
The MOSFET current limit is nominal 1.2A and guaran-
teed 1A. This restricts the maximum output current
Iomax based on the following formula:
IOMAX = 1 – ----2-I--L-- × V--V---I-O-N---
where:
IL is the inductor peak-to-peak current ripple and is
decided by:
IL = V----L-I---N-- × F--D--S--
D is the MOSFET turn-on radio and is decided by:
D = V-----O----V--–---O-V-----I--N--
FS is the switching frequency.
The following table gives typical values:
Maximum Output Current
VIN (V)
2
2
2
3
3
3
4.5
4.5
4.5
9
9
VO (V)
5
9
12
5
9
12
9
12
15
12
15
L (µH)
10
10
10
10
10
10
10
10
10
10
10
FS (kHz)
600
600
600
600
600
600
600
600
600
600
600
IOMAX (mA)
360
190
140
540
270
200
400
280
220
600
420
Component Considerations
It is recommended that CIN is larger than 10µF. Theoret-
ically, the input capacitor has ripple current of IL. Due
to high-frequency noise in the circuit, the input current
ripple may exceed the theoretical value. Larger capacitor
will reduce the ripple further.
6

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