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L5986 Schematic ( PDF Datasheet ) - STMicroelectronics

Teilenummer L5986
Beschreibung 2.5A step-down switching regulator
Hersteller STMicroelectronics
Logo STMicroelectronics Logo 




Gesamt 30 Seiten
L5986 Datasheet, Funktion
www.DataSheet4U.com
L5986
2.5 A step-down switching regulator
Preliminary Data
Features
2.5 A DC output current
2.9 V to 18 V input voltage
Output voltage adjustable from 0.6 V
250 kHz switching frequency, programmable
up to 1 MHz
Internal soft-start and inhibit
Low dropout operation: 100 % duty cycle
Voltage feed-forward
Zero load current operation
Over current and thermal protection
VFQFPN3x3-8L and HSOP8 package
Application
„ Consumer:
STB, DVD, DVD recorder, car audio, LCD TV
and monitors
„ Industrial:
PLD, PLA, FPGA, chargers
„ Networking: XDSL, modems, DC-DC modules
„ Computer:
Optical storage, hard disk drive, printers,
audio/graphic cards
„ LED driving
VFQFPN8 3x3 mm HSOP8 exposed pad
Description
The L5986 is a step down switching regulator with
3.0 A (minimum) current limited embedded power
MOSFET, so it is able to deliver up to 2.5 A
current to the load depending on the application
conditions.
The input voltage can range from 2.9 V to 18 V,
while the output voltage can be set starting from
0.6 V to VIN. Having a minimum input voltage of
2.9 V, the device is suitable also for 3.3 V bus.
Requiring a minimum set of external components,
the device includes an internal 250 kHz switching
frequency oscillator that can be externally
adjusted up to 1 MHz.
The QFN and the HSOP packages with exposed
pad allow reducing the RthJA down to 60 °C/W
and 40 °C/W respectively.
Figure 1. Application circuit
August 2008
Rev 1
This is preliminary information on a new product now in development or undergoing evaluation.
Details are subject to change without notice.
1/39
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39






L5986 Datasheet, Funktion
www.DataSEhleeect4tUri.ccoaml characteristics
L5986 - L5986A
Table 4. Electrical characteristics (continued)
Symbol
Parameter
Test condition
Values
Min Typ Max
Unit
Error amplifier
VCH High level output voltage
VCL Low level output voltage
IFB Bias source current
IO SOURCE Source COMP pin
IO SINK
GV
Sink COMP pin
Open loop voltage gain
VFB < 0.6 V
VFB > 0.6 V
VFB = 0 V to 0.8 V
VFB = 0.5 V,
VCOMP = 1 V
VFB = 0.7 V,
VCOMP = 1 V
(2)
Synchronization function
3
V
0.1
1 μA
17 mA
25 mA
100 dB
High input voltage
Low input voltage
Slave sink current
Master output amplitude
Output pulse width
VSYNCH = 2.9 V
ISOURCE = 200 μA
SYNCH floating
Input pulse width
2 3.3
V
1
0.7 0.9 mA
3.0 V
110
70
ns
Protection
IFBDISC
TSHDN
FB disconnection source
current
Thermal shutdown
Hysteresis
1 μA
150
°C
30
1. Specification referred to TJ from -40 to +125 °C. Specification in the -40 to +125 °C temperature range are
assured by design, characterization and statistical correlation.
2. Guaranteed by design.
6/39

6 Page









L5986 pdf, datenblatt
www.DataSFhuenetc4Uti.oconmal description
L5986 - L5986A
5.4 Over-current protection
The L5986 implements the over current protection sensing current flowing through the
power MOSFET. Due to the noise created by the switching activity of the power MOSFET,
the current sensing is disabled during the initial phase of the conduction time. This avoids an
erroneous detection of a fault condition. This interval is generally known as “masking time”
or “blanking time”. The masking time is about 200 ns.
When the over current is detected, two different behaviors are possible depending on the
operating condition.
1. Output voltage in regulation. When the over current is sensed, the power MOSFET is
switched off and the internal reference (VREF), that biases the non-inverting input of the
error amplifier, is set to zero and kept in this condition for a soft-start time (TSS, 2048
clock cycles). After this time, a new soft-start phase takes place and the internal
reference begins ramping (see Figure 7.a).
2. Soft-start phase. If the over current limit is reached the power MOSFET is turned off
implementing the pulse by pulse over current protection. During the soft-start phase,
under over current condition, the device can skip pulses in order to keep the output
current constant and equal to the current limit. If at the end of the “masking time” the
current is higher than the over current threshold, the power MOSFET is turned off and it
will skip one pulse. If, at the next switching on at the end of the “masking time” the
current is still higher than the threshold, the device will skip two pulses. This
mechanism is repeated and the device can skip up to seven pulses. While, if at the end
of the “masking time” the current is lower than the over current threshold, the number of
skipped cycles is decreased of one unit. At the end of soft-start phase the output
voltage is in regulation and if the over current persists the behavior explained above
takes place. (see Figure 7.b)
So the over current protection can be summarized as an “hiccup” intervention when the
output is in regulation and a constant current during the soft-start phase. If the output is
shorted to ground when the output voltage is on regulation, the over current is triggered and
the device starts cycling with a period of 2048 clock cycles between “hiccup” (power
MOSFET off and no current to the load) and “constant current” with very short on-time and
with reduced switching frequency (up to one eighth of normal switching frequency). See
Figure 31. for short circuit behavior.
12/39

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