st755 adjustable inverting negative output current mode pwm regulators ? september 2000 n 2.7v to 11v input to adjustable negative output conversion n 1w guaranteed output power (v in >4.5v, t 70 o c) n 68% typ. efficency at 6v n very low quiescent current: 1.2ma in on mode, 10 m a in shut down mode n soft start n very low noise output n 160khz fixed frequency oscillator n 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 m 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. note 1: the input to output differential voltage is limited to v cc +|v out |<12.7v connection diagram and (top view) pin connections pin no symbol name and function 1 shdn shut-down control (v cc = on, gnd=shutdown) 2v ref reference output voltage (1.25v) 3 ss soft start 4 cc compensation input 5v out negative output voltage 6 gnd ground 7 lx switch output 8v 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 cc dc input voltage to gnd (note1) -0.3 to 12 v shutdown voltage, ss voltage, cc voltage - 0.3 to v cc +0.3 v v lx switch voltage (lx to v cc ) -12.5 to +0.3 v v fb feedback voltage (v out to gnd) -11 to +0.9 v i lx peak switch current 2 a p tot power dissipation (at 70 o c) (for dip-8 ) (for so-8 ) 725 470 mw t op operating ambient temperature range -40 to 85 o c t stg storage temperature range -55 to 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 in 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 off standby current no load, v shdn = 0v 10 100 m a i sc short circuit current 1.2 a d v o line regulation v cc = 4 to 6.2 v 0.1 %/v d 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 d v ref reference dritft t a = t min to t max 50 ppm/ o c r dson lx on resistance 0.7 w i leak lx leakage current v ds = 10 v 1 m a i sh shutdown pin current 1 m a v il shutdown input low threshold 0.25 v v ih shutdown input high threshold 2v f o oscillator frequency 160 khz n power efficency i o = 100 ma 68 % cc compensation pin impedance 7.5 k w 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 w - 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 w and 20k w . 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 w 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 m 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 m 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 m fr1 130 k w c2 100 m fr2 300 k w c3 82 pf r3 10 k w c4 1 m fr4 40.7k w c5 10 m fr5 10 k w 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
figure 11: soft start delay time figure 9: switch current limit vs soft start voltage figure 10: reference voltage vs temperature typical operating characteristics (continued) st755 6/9
dim. mm inch min. typ. max. min. typ. max. a3.3 0.130 a1 0.7 0.028 b 1.39 1.65 0.055 0.065 b1 0.91 1.04 0.036 0.041 b0.5 0.020 b1 0.38 0.5 0.015 0.020 d9.80.386 e8.8 0.346 e2.54 0.100 e3 7.62 0.300 e4 7.62 0.300 f7.10.280 i4.80.189 l3.3 0.130 z 0.44 1.6 0.017 0.063 p001f plastic dip-8 mechanical data st755 7/9
dim. mm inch min. typ. max. min. typ. max. a1.750.068 a1 0.1 0.25 0.003 0.009 a2 1.65 0.064 a3 0.65 0.85 0.025 0.033 b 0.35 0.48 0.013 0.018 b1 0.19 0.25 0.007 0.010 c 0.25 0.5 0.010 0.019 c1 45 (typ.) d 4.8 5.0 0.188 0.196 e 5.8 6.2 0.228 0.244 e1.27 0.050 e3 3.81 0.150 f 3.8 4.0 0.14 0.157 l 0.4 1.27 0.015 0.050 m0.60.023 s 8 (max.) 0016023 so-8 mechanical data st755 8/9
information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specification mentioned in this pu blication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectron ics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicr oelectronics. the st logo is a registered trademark of stmicroelectronics ? 2000 stmicroelectronics C printed in italy C all rights reserved stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco singapore - spain - sweden - switzerland - united kingdom - u.s.a. http://www.st.com . st755 9/9
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