A Quick Boosting Charge Pump Circuit for High Density and Low Voltage Flash Memories

Abstract

I n t roduct ion T h e demand o f f a s t programmable f l a s h memorfes h a s been d r a s t i c a l l y i n c r e a s i n g f o r t h e replacem c n t of hard d isks by t h e semiconductor mcmories. A s i n g l e power s u p p l y EEPROM s u c h as a N A N D EEPRON [ l ] requi res t o g e n e r a t e a high vol tage on c h i p f o r i t s programming. The r i s i n g time f o r a high vol tage more s igni f icant ly occuples t h e t o t a l programming time, a c c o r d i n g as a h i g h e r d e n s i t y f lash memory increases Its i n t e r n a l c a p a c i t a n c e of t h e w i r i n g s a n d wel l s , and as t h e power s u p p l y moves t o w a r d t h e low v o l t a g e , T h i s p a p e r is devoted t o proposing a new c h a r g e pump c i r c u i t f o r a high dens i ty and low vol tage f l a s h memory which s h o u l d g c n e r a t e a high vol tage on chip v e r y fas t . Conccpt for a New Charge Pump Schcmc F o r t h e c o n v e n t i o n a l c h a r g e pump c i r c u i t . t h e number of s t a g e s connec ted in s e r i e s between t h e power s u p p l y and t h e o u t p u t o f t h e c h a r g e pump c i r c u i t is f i x e d , a s i l l u s t r a t e d In Flg.1 [ Z ] . This flxed number of s t a g e s i s so deslgned as to genera t e a c e r t a i n high v o l t a g e r e q u l r e d f o r programming. However , t h e c h a r g e t r a n s f e r e f f i c i e n c y I i 'P / ICC f o r t h e c h a r g e pump c i r c u i t i s g iven by l / ( n t l ) , where I C C and I P P a re t h e mean input and o u t p u t c u r r e n t s f o r t h e c h a r g e pump c i r c u i t . r e s p e c t i v e l y , and n is t h e number o f s t a g e s connected i n s e r i e s between t h e power supply and t h e o u t p u t of t h e c h a r g e pump c i r c u i t . As a r e s u l t , many s t a g e s connected In s e r i e s are so redundant Lhat t h e convent ional charge pump c i r c u i t no t o n l y c o n s u m e s t o o much power b u t a l s o l o w e r s t h e c h a r g e t r a n s f e r eff ic iency while t h e boosted v o l t age is not much higher than t h e power supply v o l t a g e i n t h e b e g i n n l n g o f o p e r a t i o n . Thereby , I t t a k e s longer time f o r t h e convent ional charge pump c l r c u l t t o g e n e r a t e a des i red v o l t a g e f o r programming when a h i g h e r d e n s i t y EEPROM i n c r e a s e s t h e l o a d c a p a c l t a n c e f o r t h e c h a r g e pump c i r c u l t and t h e power supply vol tage Is lowered. The proposed c h a r g e pump scheme c o n c e p t u a l l y presented in Flg.2 can change t h e number of s t a g e s and t h e capaci tance used f o r c h a r g e pumping. While t h c o u t p u t v o l t a g e I s n o t much h i g h e r t h a n t h e power supply v o l t a g e in t h e beginnlng, t h e number o f s t a g e s I s c o n t r o l l e d t o b e small a n d t h e capaci tance f o r c h a r g e pumping t o b e l a r g e so as t o increase t h e charge t r a n s f e r eff ic iency. According as t h e o u t p u t v o l t a g e I s b o o s t e d , t h e number o f s t a g e s I s i n c r e a s e d and t h e c a p a c i t a n c e f o r c h a r g c pumping 1s d e c r e a s e d s t e p by s t e p . A s a r e s u l t , t h e p r o p o s e d c h a r g e pump scheme o f f e r s a high c h a r g e t r a n s f e r e f f l c l e n c y and r e d u c e s t h e r is ing t i m e of t h e o u t p u t vol tage. 'rhe c o n c e p t f o r a proposed c h a r g e pump schemc prcscntcd above has been achleved by a p a r a l l e l to s e r i a l c o n v e r s i o n s w i t c h e s (SWl.SW2) which c a n chnngc t h e conncct lon between ne lghbor lng stages, a s shown i n F ig .3 . FIg.4 p r e s e n t s t h e c o n c r e t e charge pump c i r c u i t . Thesc two s w i t c h e s a r e c o n -