ST777/778/779
LOW VOLTAGE INPUT, 3-3.3V/5V/ADJUSTABLE OUTPUT
DC-DC CONVERTER WITH SYNCHRONOUS RECTIFIER
s
s
s
s
s
s
s
s
s
1V TO 6V INPUT GUARANTEES START-UP
UNDER LOAD
MAXIMUM OUTPUT CURRENT OF 300mA
(778 OR 779 ADJUSTED TO 3V)
LOAD FULLY DISCONNECTED IN
SHUTDOWN
TYPICAL EFFICIENCY OF 82%
INTERNAL 1A POWER SWITCH AND
SYNCHRONOUS RECTIFIER
ADJUSTABLE CURRENT LIMIT ALLOWS
LOW-COST INDUCTORS
SUPPLY CURRENT OF 270µA (NO LOAD)
SHUTDOWN SUPPLY CURRENT 20µA
PACKAGE AVAILABLE: DIP-8 AND SO-8
DIP-8
DESCRIPTION
The ST777/778/779 are dc-dc converters that
step-up from low voltage inputs requiring only
three external components, an inductor (typically
22µH) and two capacitors. The device include a
Sinchronous Rectifier that eliminates the need for
an external catch diode, and allows regulation
even when the input is greater than the output.
SCHEMATIC DIAGRAM
V
IN
O
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b
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P
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od
DELAY
TIMER
s)
t(
bs
-O
22µH
100µF
Unlike others step-up DC-DC converters the
ST777/778/779’s Sinchronous Rectifier turns off in
the shutdown mode, fully disconnecting the output
from the source. This eliminates the current drain
associated with conventional step-up converters
when off or in shutdown. Supply current is 270µA
under no load and only 20µA in stand by mode.
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SO-8
R
LIM
2
IN
1
5
I
LIM
L
X
ACTIVE RECTIFIER
t
OFF
SWITCH
DRIVER
DELAY
TIMER
OUT 6
V
O
100µF
t
ON
1 : N
RECTIFIER
CONTROL
PGND 4
SEL 8
VREF
1.25V
7 SHDN
SHUTDOWN
CONTROL
AGND
3
October 2002
1/11
ST777/778/779
ABSOLUTE MAXIMUM RATINGS
Symbol
V
CC
LX
OUT, SHDN
AGND to PGND
FB
P
TOT
T
STG
T
OP
Parameter
DC Input Voltage to GND
Switch off Pin Voltage
Switch on Pin Voltage
Output, Shutdown Voltage
Analog and Power Ground
FB Pin Voltage
Continuous Power Dissipation (at T
A
= 85°C) DIP-8
Continuous Power Dissipation (at T
A
= 85°C) SO-8
Storage Temperature Range
Operating Ambient Temperature Range
Value
-0.3 to +7
-0.3 to +7
30 sec short to IN or OUT
-0.3 to +7
-0.3 to +0.3
-0.3 to (OUT+0.3)
550
344
-40 to 150
0 to 85
Unit
V
V
V
V
V
mW
°C
°C
Absolute Maximum Ratings are those values beyond which damage to the device may occur. Functional operation under these condition is
not implied.
ORDERING CODES
TYPE
ST777
ST778
ST779
DIP-8
ST777ACN
ST778ACN
ST779ACN
SO-8
ST777ACD
ST778ACD
ST779ACD
CONNECTION DIAGRAM
PIN CONNECTIONS
Pin No.
1
2
3
4
5
6
7
8
SYMBOL
ILIM
IN
AGND
PGND
LX
OUT
SHDN
O
THERMAL DATA
Symbol
R
thj-amb
Parameter
Thermal Resistance Junction-ambient
DIP-8
100
SO-8
160
Unit
°C/W
so
b
te
le
r
P
Sets switch current limit input. Connect to IN for 1A current limit. A resistor from ILIM to IN
sets lower peak inductor currents.
Input from battery
Analog ground. Not internally connected to PGND.
Power ground. Must be low impedance; solder directly to ground plane or star ground.
Connect to AGND, close to the device.
Collector of 1A NPN power switch and emitter of Sinchronous Rectifier PNP.
Voltage Output. Connect filter capacitor close to pin.
Shutdown input disables power supply when low. Also disconnets load from input. Threshold
is set at V
IN
/2.
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NAME AND FUNCTION
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SEL/N.C./FB - Selection pin for 3/3.3V version (778);
- Not internally connected for 5V version (777);
- Feedback pin for adjustable version (779).
2/11
ST777/778/779
ELECTRICAL CHARACTERISTICS
(V
IN
=2.5V, C
I
= 22µF, C
O
=100µF, SHDN and ILIM connected to IN,
AGND connected to PGND, T
A
=0 to 85°C, unless otherwise specified. Typical values are referred at
T
A
=25°C)
Symbol
V
START
V
IN(MAX)
V
O
Parameter
Start up Voltage
Maximum Input Voltage
Output Voltage ST777 779
(set to 5V), (Note 3)
Output Voltage ST778
(Note 3)
Test Conditions
I
LOAD
< 10mA, T
A
= 25°C (Note 1)
(Note 1,2)
I
LOAD
≤
30mA, V
IN
=
1.1V to 5V or
I
LOAD
≤
80mA, V
IN
=
1.8V to 5V or
I
LOAD
≤
130mA, V
IN
=
2.4V to 5V
SEL=0V
I
LOAD
≤
50mA,V
IN
=1.1V
to 3.3V or
I
LOAD
≤
210mA,V
IN
=1.8V
to 3.3V or
I
LOAD
≤
300mA,V
IN
=2.4V
to 3.3V
SEL=OPEN
I
LOAD
≤
30mA, V
IN
=1.1V
to 3V or
I
LOAD
≤
210mA, V
IN
=1.8V
to 3V or
I
LOAD
≤
300mA, V
IN
=2.4V
to 3V
Output Voltage Range
ST779
No Load Supply Current
Shutdown Supply Current
Shutdown Input Current
Efficiency
Shutdown Input Threshold
Current Limit
(Note 4)
I
LOAD
= 0 mA, (Switch ON) (Note 5)
SHDN=0V, (Switch OFF)
SHDN = 0 to V
IN
SHDN = V
IN
to 5V
I
LOAD
=100mA
V
IN
=1V to 6V
Min.
6
4.8
5.0
5.2
Typ.
Max.
1
Unit
V
V
V
3.17
3.30
3.43
V
2.88
3.00
3.12
V
2.7
I
IN
I
SHDN
I
IN SHDN
υ
V
IH
I
LIM
I
LIM TEMPCO
Current Limit Temperature
Coefficient
Minimum Switch Off Time
t
OFFMIN
t
ONMAX
Maximum Switch ON Time
V
CESAT NPN
Switch saturation Voltage
V
CESAT NPN
Rectifier Forward Drop
b
O
so
V
FB
I
FB
I
LX
te
le
r
P
uc
od
s)
t(
V
IN
=1V
V
IN
=2.5V
V
IN
=1.8V
b
-O
so
te
le
r
P
od
20
15
12
82
1.0
-0.3
1.2
4.5
6.5
15
0.25
0.33
0.5
0.18
0.22
0.4
270
s)
t(
uc
6.5
35
100
40
V
µA
µA
nA
µA
%
V
A
V
IN
/2 + 0.25
%/°C
µs
I
SW
=400mA
I
SW
=600mA
I
SW
=1000mA
I
SW
=400mA
I
SW
=600mA
I
SW
=1000mA
V
V
Error Comparator Trip Point ST779, over operating input
voltage (Note 6)
FB Pin Bias Current
ST779, V
FB
=1.3V
Switch Off Leakage Current
Rectifier Off Leakage
Current
1.23±2%
50
0.1
0.1
V
nA
µA
µA
Note 1:
Output in regulation, V
OUT
= V
OUT
(nominal)
± 4%.
Note 2:
At hight V
IN
to V
OUT
differentials, the maximum load current is limited by the maximum allowable power dissipation in the package.
Note 3:
Start-up guaranteed under these load conditions.
Note 4:
Minimum value is production tested. Maximum value is guaranteed by design and is not production tested.
Note 5:
In the ST779 supply current depends on the resistor divider used to set the output voltage.
Note 6:
V
OUT
is set to a target value of +5V by 0.1% external feedback resistors. V
OUT
is measured to be 5V±2.5% to guarantee the error
comparator trip point.
3/11
ST777/778/779
TYPICAL APPLICATION CIRCUIT
1
ILIM
SEL / N.C. / FB
8
R2
Vin
1÷6V
2
V+
C1
22µF
L1
22µH
ST777
ST778
ST779
____
SHDN
7
R1
C3
3
AGND
Vout
6
C2
100µF
Vo
4
PGND
LX
5
APPLICATIONS INFORMATION
R1 and R2 must be placed only in ST779
applications to set the output voltage according to
the following equation:
V
OUT
= (1.23) [(R1+R2)/R2]
and to simplify the resistor selection:
R1 = R2 [(V
OUT
/1.23)-1]
It is possible to use a wide range of values for R2
(10KΩ to 50KΩ) with no significant loss of
accuracy thanks to the very low FB input current.
To have 1% error, the current through R2 must be
at least 100 times FB’s bias current.
When large values are used for the feedback
resistors (R1>50KΩ), stray output impedance at
FB can incidentally add "lag" to the feedback
response, destabilizing the regulator and creating
a larger ripple at the output. Lead lengths and
circuit board traces at the FB node should be kept
short. Compensate the loop by adding a "lead"
compensation capacitor (C3, 100pF to 1nF) in
parallel with R1.
The typical value of the L1 inductor is 22µH,
enough for most applications. However, are also
suitable values ranging from 10µF to 47µF with a
saturation rating equal to or greater than the peak
switch -current limit.
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Efficiency will be reduced if the inductor works
near its saturation limit, while will be maximized
using an inductor with a low DC resistance,
preferably under 0.2Ω.
Connecting ILIM to V
IN
the maximum LX current
limit (1A) is set. If this maximum value is not
required is possible to reduce it connecting a
resistor between ILIM and V
IN
(See Figure 16 to
choose the right value). The current limit value is
misured when the switch current through the
inductor begins to flatten and does’nt coincide with
the max short circuit current.
Even if the device is designed to tolerate a short
circuit without any damage, it is strictly
recommended to avoid a continuos and durable
short circuit of the output to GND.
To achieve the best performances from switching
power supply topology, particular care to layout
drawing is needed, in order to minimize EMI and
obtain low noise. Moreover, jitter free operation
ensures the full device functionality. Wire lengths
must be minimized, filter and by-pass capacitors
must be low ESR type, placed as close as
possible to the integrated circuit. Solder AGND
and PGND pins directly to a ground plane.
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4/11
ST777/778/779
TYPICAL CHARACTERISTICS
(unless otherwise specified T
j
= 25°C, C
I
=22µF, C
O
=100µF)
Figure 1 :
Output Voltage vs Temperature
Figure 4 :
Efficiency vs Input Voltage
Figure 2 :
Output Voltage vs Temperature
Figure 5 :
Efficiency vs Output Current
Figure 3 :
Efficiency vs Temperature
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Figure 6 :
Efficiency vs Low Output Current
5/11
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