ST755
ADJUSTABLE INVERTING NEGATIVE OUTPUT
CURRENT MODE PWM REGULATORS
September 1998
s
2.7V TO 11V INPUT TO ADJUSTABLE
NEGATIVE OUTPUT CONVERSION
s
1W GUARANTEED OUTPUT POWER
(V
in
>4.5V, T
70
o
C)
s
68% TYP. EFFICENCY AT 6V
s
VERY LOW QUIESCENT CURRENT: 1.2mA
IN ON MODE 10
A IN SHUT DOWN MODE
s
SOFT START
s
VERY LOW NOISE OUTPUT
s
160KHz FIXED FREQUENCY OSCILLATOR
s
MIXED BIPOLAR-CMOS TECHNOLOGY
DESCRIPTION
The
ST755
is
an
adjustable
inverting
switch-mode
DC-DC rergulator with
internal
Power MOSFET that generaters an adjustable
negative output from a voltage input of 2.7V to
11V, output
current guaranteed at 200mA (for
BLOCK DIAGRAM
DIP-8
SO-8
V
in
>4.5V, V
out
= -5V and 0
o
C
Ta
70
o
C) and
275mA (typical value at T
a
= 25
o
C, V
out
= -5V).
A logic controlled shut down pin that interfaces
directly with microprocessor reduces supply
current to only 10
A. Input to Output differential
voltage is limited to V
in
+|V
out
|<12.7V. No load
supply current is 1.2mA.
1/9
Absolute Maximum Rating are those values beyond which damage to the device may occur.
Functional operation under these condition is not implied.
CONNECTION DIAGRAM AND (top view)
PIN CONNECTIONS
Pin No
Symbol
Name and Fun ction
1
SHDN
SHUT-DOWN Control (V
CC
= On, GND=Shutdown)
2
V
ref
Reference Output Voltage (1.25V)
3
SS
Soft Start
4
CC
Compensation Input
5
V
out
Negative Output Voltage
6
G ND
Ground
7
LX
Switch Output
8
V
CC
Positive Supply-Voltage Input
ORDERING NUMBERS
Type
DIP-8
SO-8 (Tube)
SO-8 (T&R)
ST755
ST755CN
ST755CD
ST755CD-TR
ABSOLUTE MAXIMUM RATINGS
Symbol
Parameter
Value
Unit
V
c c
DC Input Voltage to GND
-0.3 t o 12
V
SHDN
Shutdown Voltage
-0.3 t o V
CC
+0.3
V
Other Input Voltage
-0.3 t o V
CC
+0.3
V
I
LX
Peak Switch Current
2
A
P
t ot
Power Dissipation (at 70
o
C) (for DIP-8)
(for SO-8)
725
470
mW
T
o p
Operating Ambient Temperature Range
-40 to 85
o
C
T
stg
Storage Temperature Range
-55 t o 150
o
C
ST755
2/9
ELECTRICAL CHARACTERISTICS (Refer to the test circuits, V
CC
= 5V, V
out
= -5.25 to -4.75V,
I
load
= 0mA, T
a
= T
min
to T
max
unless otherwise specified. Typical Value are referred at T
a
= 25
o
C)
Symbol
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
V
I N
Input Voltage
2.7
11
V
V
o
Output Voltage
I
o
= 0 mA to 200 mA V
CC
= 4.5 to 6.2 V
-5.25
-5
-4. 75
V
I
o
Output Current
V
CC
= 4.5 to 6.2 V T
a
= 0
o
C to 70
o
C
V
CC
= 4.5 to 6.2 V T
a
= -40
o
C to 85
o
C
V
CC
= 4 V
V
out
= -5 V
V
CC
= 2.7 V
V
out
= -5 V
200
175
275
175
125
mA
mA
mA
mA
I
SUPPLY
Supply Current (Including
Switching Current)
No Load, V
SHDN
= V
CC
1. 2
3.5
mA
I
O FF
Standby Current
No Load,
V
SHDN
= 0V
10
100
A
I
SC
Short Circuit Current
1. 2
A
V
o
Line Regulation
V
CC
= 4 to 6.2 V
0. 1
%/V
V
o
Load Regulation
I
o
= 0 mA to 200 mA
0.003
%/mA
V
REF
Reference Voltage
T
a
= 25
o
C
1.18
1.25
1.32
V
V
REF
Reference Dritft
T
a
= T
min
to T
max
50
ppm/
o
C
R
DSON
LX On Resistance
0. 7
I
LEAK
LX Leakage Current
V
DS
= 10 V
1
A
I
SH
Shutdown Pin Current
1
A
V
il
Shutdown Input Low Threshold
0.25
V
V
ih
Shutdown Input High
Threshold
2
V
f
o
Oscillator Frequency
160
KHz
Power Efficency
I
o
= 100 mA
68
%
CC
Compensation Pin Impedance
7. 5
K
Do not overload or short the Output to Ground. If the above conditions are observerd, the device may be damaged.
APPLICATION INFORMATION
The ST755 is an IC developed for voltage
conversion from an input voltage ranging from
+2.4V to 11V to a regulated adjustable negative
output limited by |V
out
|
12.7V-V
IN
. The circuit
adopts a current-mode PWM control scheme to
achieve good efficiency , high stability and low
noise performance. The figure in the first page
shown the detailed block diagram of the device.
ST755 is realized in a BCD technology in order to
achieve high temperature stability, the best
REFERENCE precision , a very low quiescent
current and jitter free operations. The final stage
is built around a 0.7
- 2A P-Channel Power
MOS. A fraction of the output current is splitted
out for current detection.Internal clock frequency
is fixed to 160KHz. Error amplifier drives the
PWM comparator in order to keep 0V on the CC
input. So R
3
and R
4
resistors are calculated by
the following formulae R
4
= (|V
out
|/V
ref
)*R
3
(see fig
1). For R
3
can be choosen any value between
2K
and 20K
. Soft-Start (SS) input is a voltage
dependent-output current limit (see figure 9,
Switch Current Limit vs. SS Input Voltage). SS
pin is internally pulled to V
ref
through a 1.2 M
resistor. Applying an appropiate capacitor at SS
input is
possible to obtain a soft-start current
limitation during power up. Forcing Soft-Start (SS)
input to a lower voltage through a resistive
voltage driver (R
1
and R
2
), the maximum LX
curent limit can be lowered according the
diagram showed in figure 9. When SHDN input is
low, the total current consumption is reduced to
10
A.
ST755
3/9
APPLICATION CIRCUIT
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, jitter free operation moreover, it
ensures the full device functionality. Layout
design proposed on demoboard (see picture 2)
helps to lower the developing time.
Wire lengths must be minimized, filter and
by-pass capacitors C
1
, C
2
and C
3
must be low
ESR type, placed as close possible to the
integrated circuit. The 10
H inductor must be
chosen built on a core, taking care that saturation
current should be higher than the peak LX switch
current. See the PEAK INDUCTOR CURRENT
vs. LOAD CURRENT graph ( figure 6)
Figure 1: TYPICAL APPLICATION CIRCUIT
Figure 2: Printed Demoboard
Symbol
Pin
V
CC
1
V
out
2
SHDN
3
GND
4
Component Values
Capacitor
Value
Unit
Resistor
Value
Unit
C1
47
F
R1
130
K
C2
100
F
R2
300
K
C3
82
pF
R3
10
K
C4
1
F
R4
40.7
K
C5
10
F
R5
10
K
C6
47
pF
(*) R1 and R2 can be omitted for Iout<200mA.
(**) C6: Very low noise but poor transient and load response speed.
(***) C3 (alternative to C6): faster transient and load response.
ST755
4/9
Figure 5: Efficency vs Load Current
Figure 7: Switch ON Resistance vs Supply
Voltage
Figure 6: Peak InductorCurrent vs Load Current
Figure 8: Oscillator Frequency vs Temperature
& Supply Voltage
Figure 3: Load Current vs Supply Voltage
Figure 4: Load Current vs Supply Voltage
TYPICAL OPERATING CHARACTERISTICS
ST755
5/9
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