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ST755CD-ST755CD-TR
ADJUSTABLE INVERTING NEGATIVE OUTPUT CURRENT MODE PWM REGULATORS
ST755ADJUSTABLE INVERTING NEGATIVE OUTPUT
CURRENT MODE PWM REGULATORS
September 2000 2.7V TO 11V INPUT TO ADJUSTABLE
NEGATIVE OUTPUT CONVERSION 1W GUARANTEED OUTPUT POWER
(Vin>4.5V, T≤70oC) 68% TYP. EFFICENCY AT 6V VERY LOW QUIESCENT CURRENT: 1.2mA
IN ON MODE, 10μA IN SHUT DOWN MODE SOFT START VERY LOW NOISE OUTPUT 160KHz FIXED FREQUENCY OSCILLATOR 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 DIAGRAMVin>4.5V, Vout = -5V and 0o C ≤ Ta ≤ 70o C) and
275mA (typical value at Ta = 25 o C, Vout = -5V).
A logic controlled shut down pin that interfaces
directly with microprocessor reduces supply
current to only 10μA. Input to Output differential
voltage is limited to Vin+|Vout|<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.
Note 1: The input to output differential voltage is limited to VCC+|VOUT|<12.7V
CONNECTION DIAGRAM AND (top view)
PIN CONNECTIONS
ORDERING NUMBERS
ABSOLUTE MAXIMUM RATINGS
ST7552/9
ELECTRICAL CHARACTERISTICS (Refer to the test circuits, VCC = 5V, Vout = -5.25 to -4.75V, Iload = 0mA, Ta = Tmin to Tmax unless otherwise specified. Typical Value are referred at Ta = 25 oC)
Do not overload or short the Output to Ground. If the above conditions are observerd, the device may be damaged.
APPLICATION INFORMATIONThe 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 |Vout| ≤ 12.7V-VIN. 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 0.7Ω - 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 R3 and R4 resistors are calculated by
the following formulae R4 = (|Vout|/Vref)*R3 (see fig
1). For R3 can be choosen any value between
2KΩ and 20KΩ. 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 Vref through a 1.2 MΩ
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 (R1 and R2), 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
10μA.
ST7553/9
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 C1, C2 and C3 must be low
ESR type, placed as close possible to the
integrated circuit. The 10μH 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 DemoboardComponent Values
(*) 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.
ST7554/9
Figure 5: Efficency vs Load Current
Figure 7: Switch ON Resistance vs SupplyVoltage
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
ST7555/9
Figure 11: Soft Start Delay Time
Figure 9: Switch Current Limit vs Soft StartVoltage
Figure 10: Reference Voltage vs temperature
TYPICAL OPERATING CHARACTERISTICS (continued)
ST7556/9
