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PDF ILC6391 Data sheet ( Hoja de datos )
| Número de pieza | ILC6391 | |
| Descripción | (ILC6390 / ILC6391) SOT-89 Step-Up PFM Switcher | |
| Fabricantes | Fairchild Semiconductor | |
| Logotipo |  |
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ILC6390/91
SOT-89 Step-Up PFM Switcher with Auto-Load Sense
Features
• 85% conversion efficiency at 50mA out
• Start-up voltages as low as 900mV
• ±2.5% accurate outputs
• Complete switch design with only 3 external components
• Automatically senses load variations to select the optimal
duty cycle and extend conversion efficiency over a wide
range
• External transistor configuration to run as switcher
controller
• Shutdown to 0.5µA
Applications
• Cellular phones, pagers
• Cameras, video recorders
• Palmtops and PDAs
Description
50 mA boost converter using Pulse Frequency Modulation, or
PFM, technique, in 5-lead SOT-89 or a 5-lead SOT-23
package. Only 3 external components are needed to complete
the switcher design.
The ILC6390 automatically senses load variations to choose
between 55% and 75% duty cycles. Normal operation is 55%
duty at 155kHz; when load currents exceed the internal
comparator trip point, a “turbo mode” kicks in to provide
extended on-time switching (75% duty at 100kHz
oscillation).
Requiring only 30µA of supply current, the ILC6390
achieves efficiencies as high as 85% at 5V yet shuts down to
0.5µA max.
Standard voltages offered are 2.5, 3.3, and 5.0V and is
available in both a 5 lead SOT-23 and 5 lead SOT-89 package
for small footprint applications.
In addition, the ILC6391 is configured to drive an external
transistor to achieve higher power levels.
Typical Applications
L
V IN
CE
31
ILC6390CM
1 23
SD
GND
+
CL
Figures 1 & 2
L: 100µH (SUMIDA, CD-54)
SD: Diode (Schottky diode;
MATSUSHITA MA 735) VIN
CL: 16V 47µF (Tantalum
V Capacitor; NICHICON, f93)
OUT
Figures 3 & 4
CE
SD
V OUT L: 47µH (SUMIDA, CD-54)
L
V IN
3 21
ILC6391CM
45
+
CL
SD: Diode (Schottky diode;
MATSUSHITA MA735)
V IN
CL: 16V 47µF (Tantalum
Capacitor; NICHICON, F93)
Tr
R
RB: 1kΩ
GND CB: 3300pF
Tr: 2SC3279, 2SDI628G
SD CE
V OUT
3 21
L
ILC6390CP
+
CL
45
GND
L
Tr
SD
3 21
ILC6391CP
CB
4
5
RB
GND
CE
V OUT
+
CL
Rev.1.2
©2001 Fairchild Semiconductor Corporation
1 page  
ILC6390/91
The ILC6390 performs boost DC-DC conversion by control-
ling the switch element shown in the circuit below.
When the switch is closed, current is built up through the
inductor. When the switch opens, this current has to go
somewhere and is forced through the diode to the output. As
this on and off switching continues, the output capacitor
voltage builds up due to the charge it is storing from the
inductor current. In this way, the output voltage gets boosted
relative to the input. The ILC6390 monitors the voltage on
the output capacitor to determine how much and how often
to drive the switch.
In general, the switching characteristic is determined by the
output voltage desired and the current required by the load.
Specifically the energy transfer is determined by the power
stored in the coil during each switching cycle.
PL = ƒ(tON, VIN)
The ILC6390 and ILC6391 use a PFM or Pulse Frequency
Modulation technique. In this technique, the switch is always
turned on for a fixed period of time, corresponding to a fixed
switching frequency at a predefined duty cycle. For the
ILC6390 this value is 3.55msec on time, corresponding to
55% duty cycle at 155kHz. Because the inductor value,
capacitor size, and switch on-time and frequency are all
fixed, the ILC6390 in essence delivers the same amount of
power to the output during each switching cycle. This in turn
creates a constant output voltage ramp which is dependent
on the output load requirement. In this mode, the only differ-
ence between the PFM and PWM techniques is the duty
cycle of the switch.
Once the output voltage reaches the set point, the ILC6390
will shut off the switch oscillator and wait until the output
voltage drops low again, at which point it will re-start the
oscillator. As you can see in the diagram, the PFM boost
converter actually skips pulses as a way of varying the
amount of power being delivered to the output.
Switch Waveform
V SET
V OUT
Because of this, PFM is sometimes called “Pulse Skipping
Modulation.”
The chief advantage of using a PFM technique is that, at low
currents, the switcher is able to maintain regulation without
constantly driving a switch on and off. This power savings
can be 5mA or more for the ILC6390 versus the ILC6370,
and at very light loads this current difference can make a
noticeable impact on overall efficiency.
However, because the ILC6390 will skip pulses based on
load current, the effective frequency of switching may well
drop into the audio band. This means that the radiated noise
of the ILC6390 may interfere with the audio channel of the
system and additional filtering may be necessary. In addi-
tion, because the PFM on-time is fixed, it usually has higher
output ripple voltage than the PWM switcher, which dynam-
ically changes the on-time to match the load current require-
ments. [Ripple is due to the output cap constantly accepting
and storing the charge received from the inductor, and deliv-
ering charge as required by the load. The “pumping” action
of the switch produces a sawtooth-shaped voltage as seen by
the output.]
On the plus side, because pulses are skipped, overtone con-
tent of the frequency noise is lower than in a PWM configu-
ration. The sum of these characteristics for PFM converters
makes it the ideal choice for low-current or ultra-long runt-
ime applications, where overall conversion efficiency at low
currents is of primary concern. [For other conversion tech-
niques, please see the ILC6370/71 and ILC6380/81
datasheets.]
Dual-Step Mode
The ILC6390 and ILC6391 have one other unique feature,
that being to automatically switch to a second switching
scheme in the presence of heavy output loading. As we men-
tioned, the standard switching scheme for these parts is a
3.55msec, 155kHz, 55% duty cycle part. However, if the
device detects that the output load increases beyond a set
point (as seen by the voltage drop on the output capacitor), it
switches in a 7.5msec, 100kHz, 75% duty cycle “turbo
mode” specifically to keep up with the increased load
demand. This switchover is seamless to the user, but will
result in a change in the output ripple voltage characteristic
of the DC-DC converter.
PFM converters are widely used in portable consumer appli-
cations not requiring a high current level and relatively unaf-
fected by audio noise. Applications such as pagers and
PDAs, which need to operate in stand-by for extended peri-
ods of time, gravitate toward the advantages of PFM since
maximum run-time is a chief differentiating element. The
ILC6390 addresses this low-current requirement, and addi-
tionally offers a “turbo” mode which maintains output regu-
lation in the presence of heavier-than-normal load currents,
and maintains 0.5mA shutdown currents.
The only difference between the ILC6390 and ILC6391
parts is that the 6391 is configured to drive an external tran-
sistor as the switch element. Since larger transistors can be
selected for this element, higher effective loads can be regu-
lated.
©2001 Fairchild Semiconductor Corporation
5
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