Datasheet LTC3525-3, LTC3525-3.3, LTC3525-5 (Analog Devices) - 10

LTC3525-3/ LTC3525-3.3/LTC3525-5
operaTion
The LTC3525 is a high performance Burst Mode operation only, synchronous boost converter requiring only three LTC3525 V LTC3525 small external components. Its simplicity and small size IN make it a high efficiency alternative to charge pump designs. SHDN SHDN R 3525 F01 It is designed to start up from a single alkaline or nickel VCNTRL cell, with input voltages as low as 1V, or from two or three 1M ZETEX ZC2811E cells (or a Li-Ion battery), with voltages as high as 4.5V. 1M VCNTRL Once started, VIN can be as low as 0.5V (depending on R > (VCNTRL/(VIN + 0.4) – 1) MΩ load current) and maintain regulation. The output voltage
Figure 1
is preset internally to either 3V, 3.3V or 5V. Peak switch current is 400mA minimum, providing regulation with load currents up to 150mA, depending on input voltage. After the SHDN pin rises, there is a short delay before switching starts. The delay is 20µs to 120µs, depending Synchronous rectification provides high efficiency opera- on input voltage (see Typical Performance Characteristics tion while eliminating the need for an external Schottky curve). diode. True output disconnect eliminates inrush current at start-up, and allows VOUT to be disconnected from VIN,
Start-Up
for zero shutdown current. A start-up oscillator allows the LTC3525 to start with input The output disconnect feature also allows the LTC3525 to voltages as low as 1V. It remains in start-up mode until maintain regulation with an input voltage equal to or greater two conditions are met. VOUT must exceed VIN by at least than VOUT. Note, however, that the synchronous rectifier 0.2V typical and either VIN or VOUT must be greater than is not enabled in this mode resulting in lower efficiency 1.8V typical. and reduced output current capability. During start-up, the synchronous rectifier is not enabled, The operating quiescent current is only 7µA typical, allow- and the internal P-channel synchronous rectifier acts as ing the converter to maintain high efficiency at extremely a follower, causing the peak voltage on SW to reach (VIN light loads. + 1V) typical. This limits inrush current by maintaining control of the inductor current when VOUT is less than VIN.
Shutdown
To reduce power dissipation in the P-channel synchronous The LTC3525 is shut down by pulling SHDN below 0.4V, rectifier when the output is shorted, a foldback feature is and made active by raising it above 1V. Although SHDN can incorporated that reduces the peak inductor current when be driven above V V IN or VOUT (up to the absolute maximum IN is more than 1.7V greater than VOUT. rating) without damage, the LTC3525 has a proprietary test mode that may be engaged if SHDN is held in the range
Normal Operation
of 0.5V to 1V higher than the greater of VIN or VOUT. If Once VOUT has increased more than 0.2V typical above the test mode is engaged, normal PWM switching action VIN, and either voltage is above 1.8V, normal operation is interrupted, which can cause undesirable operation begins, with synchronous rectification enabled. In this in some applications. Therefore, in applications where mode, the internal N-channel MOSFET connected be- SHDN may be driven above VIN, a resistor divider or other tween SW and GND stays on until the inductor current means must be employed to keep the SHDN voltage below reaches a maximum peak value, after which it is turned (VIN + 0.4V) to prevent the possibility of the test mode off and the P-channel synchronous rectifier is turned being engaged. Please refer to Figure 1 for two possible on. It stays on, delivering current to the output, until the implementations. inductor current has dropped below a minimum value at 3525fc 10 For more information www.linear.com/LTC3525 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Typical Application Related Parts