Final
Impala Linear Corporation
ILC6390/1
SOT-89 Step-Up PFM Switcher with Auto-Load Sense
General Description
50 mA boost converter using Pulse Frequency Modu-
lation, 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.
Features
w
85% conversion efficiency at 50mA out
w
Start-up voltages as low as 900mV
w
±2.5% accurate outputs
June 1996
w
Complete switch design with only 3 external components
w
Automatically senses load variations to select the optimal
duty cycle and extend conversion efficiency over a wide
range
w
External transistor configuration to run as switcher
controller
w
Shutdown to 0.5µA
Applications
w
Cellular phones, pagers
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Cameras, video recorders
w
Palmtops and PDAs
Block Diagram
L
X
V
LX
LIMITER
BUFFER
V
REF
V
DD
V
OUT
Ordering Information*
ILC6390CM-25 2.5V±2.5%
ILC6390CM-33 3.3V±2.5%
ILC6390CM-50 5.0V±2.5%
ILC6391CM-25 3.3V ±2.5% driving external transistor
V
SS
2-STEP PFM
CONTROLLED OSC
+
EXT
CE
100/155KHz
-
+
CHIP ENABLE
-
4~5mV
ILC6391CM-33 3.3V ±2.5% driving external transistor
ILC6391CM-50 5.0V ±2.5% driving external transistor
V
SS
4
Pin-Package Configurations
L
X
5
V
SS
4
EXT
5
ILC6390CP-25 2.5V±2.5%
ILC6390CP-33 3.3V±2.5%
ILC6390CP-50 5.0V±2.5%
ILC6391CP-25 3.3V ±2.5% driving external transistor
ILC6391CP-33 3.3V ±2.5% driving external transistor
ILC6391CP-50 5.0V ±2.5% driving external transistor
* Standard product offering comes in tape & reel, quantity
3000 per reel, orientation right for SOT-25, quantity 1000 per
reel orientation right for SOT-89.
SOT -25
(TOP VIEW)
1
2
3
SOT -25
(TOP VIEW)
1
2
3
CE V
DD
N/C
CE V
DD
N/C
ILC6390CM
V
SS
5
ILC6391CM
V
SS
5
L
X
4
EXT
4
SOT -89-5
(TOP VIEW)
1
2
3
SOT -89-5
(TOP VIEW)
1
2
3
N/C
V
OUT
CE
N/C
V
OUT
CE
ILC6390CP
ILC6391CP
Impala Linear Corporation
1
Electrical Characteristics ILC6390
V
OUT
=5.0V T
A
=25°C, Unless otherwise specified, V
IN
=V
OUT
X0.6, I
OUT
=50mA. See the schematic, Fig.1.
Parameter
CE “High” Current
CE “Low” Current
L
x
Limit Voltage
Efficiency
Note:
Symbol
I
CEH
I
CEL
V
LXLMT
EFFI
Conditions
V
CE
= V
OUT
x 0.95
V
OUT
= 4.75V, V
CE
=0V
V
OUT
= 4.75V, fosc> MFO1 x 2
(3)
Test Circuit of Figure 1
Min
Typ
Max
0.25
-0.25
Units
µA
µA
V
%
0.7
85
1.1
1. The Schottky diode (S.D.), in figure 1 must be type MA735, with Reverse current (I
R
) < 1.0µA at reverse voltage (V
R
)=10.0V
2. “Supply Current 1” is the supply current while the oscillator is continuously oscillating. In actual operation
the oscillato periodically operates which results in less average power consumption.
The current that is actually provided by external V
IN
source is represented by “No-Load Input Current”
3. The switching frequency is determined by the delay time of the internal comparator and MFO1, which sets the min. on-time
Electrical Characteristics ILC6391
V
OUT
=5.0V T
A
=25°C, Unless otherwise specified, V
IN
=V
OUT
X0.6, I
OUT
=50mA. See the schematic, Fig.2
Parameter
Output Voltage
Input Voltage
Operation Startup Voltage
Operation Hold Voltage
Supply Current 1
(1)
Supply Current 2
EXT ”High” On-Resistance
EXT ”Low” On-Resistance
Duty Ratio 1
Duty Ratio 2
Maximum Oscillation Freq. 1
Maximum Oscillation Freq. 2
Stand=by Current
CE “High” Voltage
CE “Low” Voltage
CE “High” Current
CE “Low” Current
Efficiency
Note:
Symbol
V
OUT
V
IN
V
ST
V
HLD
I
DD
1
I
DD
2
R
EXTH
R
EXTL
DUTY 1
DUTY 2
MFO 1
MFO 2
I
STB
V
CEH
V
CEL
I
CEH
I
CEL
EFFI
I
OUT
=1mA
I
OUT
=1mA
V
OUT
= 4.75V
V
OUT
= 5.5V
Conditions
Test Circuit of Figure 2
Min
4.875
Typ
5.000
Max
5.125
10
Units
V
V
V
V
0.80
0.70
31.7
2.4
50
50
70
50
85
153
75
55
100
180
0.9
63.4
4.8
75
75
80
60
115
207
0.5
µA
µA
Ω
Ω
%
%
KHz
KHz
µA
V
V
OUT
= 4.75V, V
EXT
=V
OUT
-0.4
V
OUT
= 4.75V, V
EXT
=0.4
V
OUT
= 4.75V, Measuring of EXT waveform
V
IN
= V
OUT
x 0.95, Iout = 1mA, Measuring of EXT
High state
V
OUT
= 4.75V, 75% duty
V
IN
= V
OUT
x 0.95, 55% duty
V
OUT
= 4.75V
V
OUT
= 4.75V, Existence of EXT Oscillation
V
OUT
= 4.75V Disappearance of EXT Oscillation
V
CE
= V
OUT
= 4.75V
V
OUT
= 4.75V, V
CE
=0V
Test Circuit of Figure 2
0.75
0.20
0.25
-0.25
85
V
µA
µA
%
1. “Supply Current 1” is the supply current while the oscillator is continuously oscillating.
periodically operates which results in less average power consumption.
In actual operation the oscillator
Impala Linear Corporation
3
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.
P
L
= ƒ(t
ON
, V
IN
)
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 3.55µsec 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.
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 5µA 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
addition, because the PFM on-time is fixed, it usually has
higher output ripple voltage than the PWM switcher, which
dynamically changes the on-time to match the load current
requirements. [Ripple
is due to the output cap constantly
accepting and storing the charge received from the induc-
tor, and delivering 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
content of the frequency noise is lower than in a PWM con-
figuration. The sum of these characteristics for PFM con-
verters makes it the ideal choice for low-current or ultra-
long runtime applications, where overall conversion effi-
ciency at low currents is of primary concern.
[For other
conversion techniques, 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
mentioned, the standard switching scheme for these parts
is a 3.55µsec, 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 capac-
itor), it switches in a 7.5µsec, 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 charac-
teristic of the DC-DC converter.
PFM converters are widely used in portable consumer
applications not requiring a high current level and relatively
unaffected by audio noise. Applications such as pagers
and PDAs, which need to operate in stand-by for extended
periods 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
additionally offers a “turbo” mode which maintains output
regulation in the presence of heavier-than-normal load cur-
rents, and maintains 0.5µA shutdown currents.
The only difference between the ILC6390 and ILC6391
parts is that the 6391 is configured to drive an external
transistor as the switch element. Since larger transistors
can be selected for this element, higher effective loads can
be regulated.
Switch Waveform
V
SET
V
OUT
Impala Linear Corporation
5
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