Datasheet LTC3525L-3 (Analog Devices) - 6

LTC3525L-3
OPERATION
The LTC3525L-3 is a high performance Burst Mode op-
Shutdown
eration only, synchronous boost converter requiring only The LTC3525L-3 is shut down by pulling SHDN below three small external components. Its simplicity and small 0.3V, and made active by raising it above 0.88V. Although size make it a high efficiency alternative to charge pump SHDN can be driven above V designs. It is designed to start up from a single alkaline IN or VOUT (up to the absolute maximum rating) without damage, the LTC3525L-3 has a or nickel cell, with input voltages as low as 0.7V typical proprietary test mode that may be engaged if SHDN is held or 0.88V maximum, or from two or three cells (or a Li-Ion in the range of 0.5V to 1V higher than the greater of V battery), with voltages as high as 4.5V. Once started, V IN or IN V can be as low as 0.5V (depending on load current) and OUT. If the test mode is engaged, normal PWM switching action is interrupted, which can cause undesirable opera- maintain regulation. The output voltage is preset internally tion in some applications. Therefore, in applications where to 3V. Peak switch current is 400mA minimum, providing SHDN may be driven above V regulation with load currents up to 160mA, depending on IN, a resistor divider or other means must be employed to keep the SHDN voltage below input voltage. (VIN + 0.4V) to prevent the possibility of the test mode Synchronous rectification provides high efficiency opera- being engaged. Please refer to Figure 1 for two possible tion while eliminating the need for an external Schottky implementations. diode. True output disconnect eliminates inrush current After the SHDN pin rises, there is a short delay before at start-up, and allows VOUT to be disconnected from VIN, switching starts. The delay is 20µs to 120µs, depending for zero shutdown current. on input voltage (see Typical Performance Characteristics The output disconnect feature also allows the LTC3525L-3 curve). to maintain regulation with an input voltage equal to or greater than V
Start-Up
OUT. Note, however, that the synchronous rectifier is not enabled in this mode resulting in lower A start-up oscillator allows the LTC3525L-3 to start with in- efficiency and reduced output current capability. put voltages as low as 0.7V. It remains in start-up mode until The operating quiescent current is only 7µA typical, allow- two conditions are met. VOUT must exceed VIN by at least ing the converter to maintain high efficiency at extremely 0.2V typical and either VIN or VOUT must be greater than light loads. 1.8V typical. LTC3525L-3 VIN LTC3525L-3 SHDN SHDN R 3525L F01 VCNTRL 1M ZETEX ZC2811E 1M VCNTRL R > (VCNTRL/(VIN + 0.4) – 1) MΩ
Figure 1. Recommended Shutdown Circuits when Driving SHDN Above VIN
3525lfa 6 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Package Description Revision History Typical Applications Related Parts