Datasheet SP6690 (Sipex) - 4

FUNCTIONAL DIAGRAM
VIN SW 5 1 R1 R2 X1 DISABLE + POWER - SET TRANSISTOR Q1 Q2 250ns FB 3 ONE-SHOT DRIVER CLEAR R3 X2 + - 52.5mV R4 0.15 GND SHDN 4 Shutdown 2 Logic
THEORY OF OPERATION General Overview:
Operation can be best understood by referring to At the end of the 250ns time period, driver the functional diagram above and the typical transistor is again allowed to turn on which application circuit on the front page. Q1 and Q2 ramps the current back up to the 350mA level. along with R3 and R4 form a band gap refer- Comparator X2 clears the latch, it’s output turns ence. The input to this circuit completes a feed- off the driver transistor, and this allows delivery back path from the high voltage output through of L1’s stored kinetic energy to C2. This switch- a voltage divider, and is used as the regulation ing action continues until the output capacitor control input. When the voltage at the FB pin is voltage is charged to the point where FB is at slightly above 1.22V, comparator X1 disables band gap (1.22V). When this condition is most of the internal circuitry. Current is then reached, X1 turns off the internal circuitry and provided by capacitor C2, which slowly dis- the cycle repeats. The SP6690 contains circuitry charges until the voltage at the FB pin drops to provide protection during start-up and while below the lower hysteresis point of X1, about in short-circuit conditions. When FB pin volt- 6mV. X1 then enables the internal circuitry, age is less than approximately 300mV, the switch turns on chip power, and the current in the off time is increased to about 1.2uS and the inductor begins to ramp up. When the current current limit is reduced to about 70% of its through the driver transistor reaches about normal value. While in this mode, the average 350mA, comparator X2 clears the latch, which inductor current is reduced and helps minimize turns off the driver transistor for a preset 250nS. power dissipation in the SP6690, the external At the instant of shutoff, inductor current is inductor and diode. diverted to the output through diode D1. During this 250nS time limit, inductor current decreases while its energy charges C2. Date: 08/31/09 SP6690 Micro Power Boost Regulator, Series White LED Driver © Copyright 2005 Sipex Corporation 4