ts 1937 boost (step - up) wled driver 1 / 8 version: c 07 sot - 2 5 general description the TS1937 is a step - up dc/dc converter specifically designed to drive white leds with a constant current. the device can drive 2 ~ 12 leds in series from a li - ion cell. series connection of the leds provides identical led currents resulting in uniform brightness and eliminating the need for ballast resistors. the output capacitor can be as small as 0.22 f, saving space versus alternative solutions. a low 95mv feedback voltage minimizes power loss for better efficiency. additional feature include over output voltage limiting when leds are disconnected. the TS1937 switches at a fixed frequency of 1.2mhz, allowing the use of tiny, low profile inductors and capacitors to minimize footprint and cost i n space consideration applications for cellular phone backlighting or other hand held equipment. application cellular phones digital still cameras mp3 players white led backlighting pdas, handheld computers features inherently matched led current high efficiency: 88 % typical drives up to 6 leds @ vin 5v drives up to 1 2 leds @ vin 12 ~15 v 36v rugged bipolar switc h fast 1.2mhz switching frequency requires only 0.22 f output capacitor pwm dimming control 1 k hz to 1 0 khz analog dimming control ordering info rmation part no. package packing ts 1937cx5 rf sot - 2 5 3kpcs / 7 reel typical application circuit pin definition : 1. sw 2. ground 3. feedback 4. ctrl 5. input
ts 1937 boost (step - up) wled driver 2 / 8 version: c 07 absolute maximum rating parameter symbol limit unit input voltage v in 20 v supply voltage (recommended) v in 2. 5 ~ 18 v fb voltage v fb 10 v s w voltage v sw 3 6 v ctrl voltage v ctrl 10 v ambient temperature range t a - 40 to +85 o c j unction temperature range t j - 40 to +125 o c electrical specifications ( ta = 25 o c , v in = v ctrl =3v, c in =1uf, c out = 0.22 uf unless otherwise noted ) function parameter symb ol test conditions min typ max units input voltage range v in 2. 5 -- 18 v feedback voltage v fb i sw = 10 0ma, 66 % duty cycle 86 95 104 mv fb pin bias current i b 10 45 100 n a -- 1. 9 2.5 ma supply current i q ctrl = 0v -- 0. 1 1 .0 ua switching frequency f rsw 0.8 1.2 1.6 m hz switch current limit i cl -- 320 -- ma maximum duty cycle d tmx 8 5 90 -- % switch vcesat v sat at isw = 2 5 0ma -- 350 -- mv switch leakage current i lkg vsw = 5 v -- 0.01 5 ? high 1. 5 -- -- v voltage enable control v ct r l low -- -- 0. 4 v ctrl pi n bias current i ct r l ctrl = 2v -- 65 -- ? thermal resistance ja -- 220 -- ?/w note: absolute maximum ratings are limits beyond which damage to the device may occur. the maximum allowable power dissipation is a function of maximum function temperature, t j(max), the junction to ambient thermal resistance, ja , and the ambient temperature. the maximum allowable, power dissipation at any ambient temperature is calculated using: p d(max) = [t j(max) - t a ]/ ja . exceeding the maximum allowable power dissipation will cause excessive die temperature. all limits at temperature extremes are guaranteed via correlation using standard statistical methods
ts 1937 boost (step - up) wled driver 3 / 8 version: c 07 functional block pin description pin function description 1 sw switching pin. this is the collector of the internal npn power switch. connect to inductor and diode. minimize the metal trace area connected to this pin to reduce emi. 2 ground ground pin. connect directly to local ground plane. 3 feedback feedback pin. reference voltage is 95 mv. connect leds and a resistor at this pin. led current is determined by the resistance and ctrl voltage. 4 ctrl shutdown pin and dimming control pin. vctrl > 1.8v generates full - scale led current vctrl < 0.4v chip is off switching from 0 . 4v to 2.0v, pwm duty cycle cont rols the led current 5 input input supply pin. bypass this pin with a capacitor as close to the device as possible
ts 1937 boost (step - up) wled driver 4 / 8 version: c 07 application information operation the ts 1937 uses a constant frequency, current mode control scheme to provide excellent line and lo ad regulation. operation can be best understood by referring to the block diagram. at the start of each oscillator cycle, the rs latch is set, which turns on the power switch q1. a voltage proportional to the switch current is added to a stabilizing ramp a nd the resulting sum is fed into the positive terminal of the pwm comparator a2. when this voltage exceeds the level at the negative input of a2, the rs latch is reset turning off the power switch. the level at the negative input of a2 is set by the error amplifier a1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 95mv. in this manner, the error amplifier sets the correct peak current level to keep the output in regulation. if the error amplif ier? output increases, more current is delivered to the output; if it decreases, less current is delivered. minimum output current the ts1 937 can regulate three series leds connected at low output currents, down to approximately 4ma from a 4.2v supply, w ithout pulse skipping, using the same external components as specified for 20 ma operation. as the current is further reduced, the device will begin skipping pulses. this will result in some low frequency ripple, although the led current remains regulated o n an average basis down to zero. soft start and current limit the internal soft start circuit minimizes the inrush current during turning on TS1937. the typical switch current is limited to about 32 0 ma by the chip . o ver voltage protection the ts 1937 has d esign an i nternal latched off open - circuit protection circuit , the additional sense pin to detect the voltage w hen the leds are disconnected from the circuit or fail open, the ts 1937 will shutdown until input condition changes to bring it out of the shutdo wn mode. inductor selection a 22mh inductor is recommended for most ts 1937 applications. although small size and high efficiency are major concerns, the inductor should have low core losses at 1.2mhz and low dcr (copper wire resistance). diode selection to maintain high efficiency, the average current rating of the schottky diode should be large than the peak inductor current, ipk. schottky diode with a low forward drop and fast switching speeds are ideal for increase efficiency in portable application. c hoose a reverse breakdown of the schottky diode large than the output voltage. a schottky diode rated at 100ma to 200ma is sufficient for most ts 1937 applications. capacitor selection the small size of ceramic capacitors makes them ideal for ts 1937 applic ations. x5r and x7r types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as y5v or z5u. a 1 f input capacitor and a 0.22 f output ca pacitor are sufficient for most .
ts 1937 boost (step - up) wled driver 5 / 8 version: c 07 application information (continue) reference design informa t i o n vin range (v) vout max. (v) l (uh) led (series) current (ma) resistor ( ) efficiency (%) 5 36 22 6pcs 20 4.75 83.5 9 36 33 11pcs 20 4.75 84.2 12 36 47 12pcs 20 4.75 84.3 15 36 68 12pcs 20 4.75 8 4 .6 led dimming control there are some different types of dimming control circuits: 1. using a pwm signal to shdn pin with the pwm signal applied to the shdn pin, the ts 1937 is turned on or off by the pwm signal. the leds operate at either zero or full current. the average led current increases proportionally with the duty cycle of the pwm signal. a 0% duty cycle wil l turn off the ts 1937 and corresponds to zero led current. a 100% duty cycle corresponds to full current. the typical frequency range of the pwm signal is 1khz to 10khz. the magnitude of the pwm signal should be higher than the minimum shdn voltage high. for some applications, the preferred method of brightness control is a variable dc voltage to adjust the led current. the dimming control using a dc voltage is shown in figure 4. as the dc voltage increases, the voltage drop on r2 increases and the voltag e drop on r1 decreases. thus, the led current decreases. the selection of r2 and r3 will make the current from the variable dc source much smaller than the led current and much larger than the fb pin bias current. for vdc range from 0v to 2v, the selection of resistors in figure 4 gives dimming control of led current from 0ma to 20 ma. dimming control using a dc voltage v dc(v) v fb iout(ma) 2 0 0 1.8 2.2 0.43 1.6 9.2 1.8 1.4 19.6 3.6 1.2 31.1 6 1 43.4 8.5 0.8 63 12.3 0.6 74 14.5 0.4 86.4 16.9 0.2 96.9 19 0 102 20
ts 1937 boost (step - up) wled driver 6 / 8 version: c 07 application information (continue) 2. using a filtered pwm signal the filtered pwm signal can be considered as a n adjustable dc voltage. it can be used to replace the variable dc voltage source in dimming control. the circuit is shown as follow: dimming control using a filtered pwm signal led current control the led current is controlled by the feedback resistor (r1). the feedback reference is 95mv. the led current is 95mv/r1. in order to have accurate led current, precision resistors are preferred (1% is recommended) . the formula and table 3 for r1 selection are shown below. r1 = 95mv/iled iled (m a) r1( ) 5 19.1 10 9.53 12 7.87 15 6.34 20 4.75 duty iout ( ma) 0% 20 20% 17.4 30% 15 40% 12.4 50% 10 60% 8 70% 6.4 80% 4.07 100% 0 pwm : 2v ; 1khz ; vin= 3.6v
ts 1937 boost (step - up) wled driver 7 / 8 version: c 07 sot - 25 mechan ical drawing marking diagram 37 = device code y = year code m = month code ( a =jan, b =feb, c =mar, d =apl, e =may, f =jun, g =jul, h =aug, i =sep, j =oct, k =nov, l =dec) l = lot code sot - 25 dimension millimeters inches dim min max min max. a+a1 0.09 1.25 0.0354 0.0492 b 0.30 0.50 0.0118 0.0197 c 0.09 0.25 0.0035 0.0098 d 2.70 3.10 0.1063 0.1220 e 1.40 1.80 0.0551 0.0709 e 1.90 bsc 0.0748 bsc h 2.40 3.00 0.09449 0.1181 l 0.35 bsc 0.0138 bsc ? 1 0 10 0 10 s1 0.95 bsc 0.0374 bsc
ts 1937 boost (step - up) wled driver 8 / 8 version: c 07 notice specifications of the products displayed herein are subject to change without notice. tsc or anyone on its behalf, assumes no responsibility or liability for any errors or inaccuracies. information contained here in is intended to provide a product description only. no license, express or implied, to any intellectual property rights is granted by this document. except as provided in tsc? terms and conditions of sale for such products, tsc assumes no liability what soever, and disclaims any express or implied warranty, relating to sale and/or use of tsc products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright, or other intellect ual property right. the products shown herein are not designed for use in medical, life - saving, or life - sustaining applications. customers using or selling these products for use in such applications do so at their own risk and agree to fully indemnify ts c for any damages resulting from such improper use or sale.
|
