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PDF L6728A Data sheet ( Hoja de datos )
| Número de pieza | L6728A | |
| Descripción | High frequency single phase PWM controller | |
| Fabricantes | STMicroelectronics | |
| Logotipo |  |
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L6728A
High frequency single phase PWM controller with Power Good
Features
■ Flexible power supply from 5 V to 12 V
■ Power conversion input as low as 1.5 V
■ 0.8 V internal reference
■ 0.8 % output voltage accuracy
■ High-current integrated drivers
■ Power Good output
■ Sensorless and programmable OCP across
low-side RDS(on)
■ OV / UV protections
■ VSEN disconnection protection
■ Oscillator internally fixed at 600 kHz
■ LS-LESS to manage pre-bias start-up
■ Adjustable output voltage
■ Disable function
■ Internal soft-start
■ VFDFPN 10 package
Applications
■ Memory and termination supply
■ Subsystem power supply (MCH, IOCH, PCI)
■ CPU and DSP power supply
■ Distributed power supply
■ General DC / DC converters
VFQFPN 10
Description
L6728A is a single-phase step-down controller
with integrated high-current drivers that provides
complete control logic and protection to realize in
a simple way general DC-DC converters by using
a compact VFDFPN 10 package.
Device flexibility allows managing conversions
with power input VIN as low as 1.5 V and device
supply voltage ranging from 5 V to 12 V.
L6728A provides simple control loop with voltage
mode EA. The integrated 0.8 V reference allows
regulating output voltages with ±0.8 % accuracy
over line and temperature variations. Oscillator is
internally fixed to 600 kHz.
L6728A provides programmable dual level over
current protection as well as over and under
voltage protection. Current information is
monitored across the low-side MOSFET RDS(on)
saving the use of expensive and space-
consuming sense resistors.
PGOOD output easily provides real-time
information on output voltage status, through
VSEN dedicated output monitor.
Table 1.
Device summary
Order codes
L6728A
L6728ATR
Package
VFDFPN 10
VFDFPN 10
Packing
Tube
Tape and reel
October 2008
Rev 1
1/32
www.st.com
1
1 page  
L6728A
Pins description and connection diagrams
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2 Pins description and connection diagrams
Figure 3. Pins connection (top view)
BOOT
PHASE
UGATE
LGATE / OC
GND
1 10
29
3 L6728A 8
47
56
PGOOD
VSEN
FB
COMP / DIS
VCC
2.1
Pin descriptions
Table 2. Pin description
Pin #
Name
Function
HS driver supply.
1 BOOT Connect through a capacitor (100 nF) to the floating node (LS-Drain) pin
and provide necessary bootstrap diode from VCC.
HS driver return path, current-reading and adaptive-dead-time monitor.
2
PHASE
Connect to the LS drain to sense RDS(on) drop to measure the output
current. This pin is also used by the adaptive-dead-time control circuitry to
monitor when HS MOSFET is OFF.
3 UGATE HS driver output. Connect directly to HS MOSFET gate.
LGATE. LS driver output. Connect directly to LS MOSFET gate.
OC over-current threshold set. During a short period of time following VCC
rising over UVLO threshold, a 10 μA current is sourced from this pin.
4 LGATE / OC Connect to GND with an ROCSET resistor greater than 5 kΩ to program OC
Threshold. The resulting voltage at this pin is sampled and held internally
as the OC set point. Maximum programmable OC threshold is 0.55 V. A
voltage greater than 0.6 V activates an internal clamp and causes OC
threshold to be set at the maximum value.
All internal references, logic and drivers are connected to this pin.
5 GND
Connect to the PCB ground plane.
Device and Drivers power supply.
6 VCC
Operative range from 5 V to 12 V. Filter with at least 1 μF MLCC to GND.
COMP. Error amplifier output. Connect with an RF - CF // CP to FB to
compensate the device control loop.
7 COMP / DIS
DIS. The device can be disabled by pushing this pin lower than 0.75 V (typ).
Setting free the pin, the device enables again.
Error amplifier inverting input.
8 FB Connect with a resistor RFB to the output regulated voltage. Output resistor
divider may be used to regulate voltages higher than the reference.
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5 Page 
L6728A
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6 Soft-start
Soft-start
L6728A implements a soft-start to smoothly charge the output filter avoiding high in-rush
currents to be required from the input power supply. The device gradually increases the
internal reference from 0 V to 0.8 V in 4.5 msec (typ.), in closed loop regulation, linearly
charging the output capacitors to the final regulation voltage. A pre-charged output voltage
will affect the soft-start duration, resulting in a reduction of this period of time (< 4 msec).
During the soft-start process all the protections but the UVP are active: the UVP becomes
active as soon as the soft-start ends up.
The device begins soft-start phase only when VCC power supply is above UVLO threshold
and over-current threshold setting phase has been completed.
6.1 Low-side-less start up (LSLess)
In order to avoid any kind of negative undershoot and dangerous return from the load during
start-up, L6728A performs a special sequence in enabling LS driver to switch: during the
soft-start phase, the LS driver results disabled (LS = OFF) until the HS starts to switch. This
avoid the dangerous negative spike on the output voltage that can happen if starting over a
pre-biased output.
If the output voltage is pre-biased to a voltage higher than the final one, the HS would never
start to switch. In this case, at the end of soft-start time, LS is enabled and discharge the
output to the final regulation value.
This particular feature of the device masks the LS turn-on only from the control loop point of
view: protections by-pass this turning ON the LS MOSFET in case of need.
Figure 4. LSLess startup (left) vs. non-LSLess startup (right)
11/32
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| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet L6728A.PDF ] | |
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