Datasheet LTC3528, LTC3528B (Analog Devices) - 8

LTC3528/LTC3528B
OPERATION (Refer to Block Diagram)
The LTC3528/LTC3528B are 1MHz synchronous boost
LOW NOISE FIXED FREQUENCY OPERATION
converters housed in an 8-lead 3mm × 2mm DFN package. With the ability to start-up and operate from inputs less
Soft-Start
than 0.88V, the devices feature fixed frequency, current The LTC3528/LTC3528B contains internal circuitry to pro- mode PWM control for exceptional line and load regula- vide soft-start operation. The internal soft-start circuitry tion. The current mode architecture with adaptive slope slowly ramps the peak inductor current from zero to its compensation provides excellent transient load response peak value of 1.5A (typical), allowing start-up into heavy and requires minimal output filtering. Internal soft-start and loads. The soft-start time is approximately 0.5ms. The internal loop compensation simplifies the design process soft-start circuitry is reset in the event of a commanded while minimizing the number of external components. shutdown or a thermal shutdown. With its low RDS(ON) and low gate charge internal N-channel MOSFET switch and P-channel MOSFET synchronous
Oscillator
rectifier, the LTC3528 achieves high efficiency over a wide An internal oscillator sets the frequency of operation to range of load current. Burst Mode operation maintains 1MHz. high efficiency at very light loads, reducing the quiescent current to 12µA. Operation can be best understood by
Shutdown
referring to the Block Diagram. The converter is shut down by pulling the SHDN pin below 0.25V, and activated by pulling SHDN above 0.88V. Although
LOW VOLTAGE START-UP
SHDN can be driven above VIN or VOUT (up to the absolute The LTC3528/LTC3528B includes an independent start-up maximum rating) without damage, the LTC3528/LTC3528B oscillator designed to operate at an input voltage of 0.70V have a proprietary test mode that may be engaged if (typical). Soft-start and inrush current limiting are provided SHDN is held in the range of 0.5V to 1V higher than the during start-up, as well as normal operating mode. greater of VIN or VOUT. If the test mode is engaged, normal PWM switching action is interrupted, which can cause When either VIN or VOUT exceeds 1.6V typical, the IC enters undesirable operation in some applications. Therefore, normal operating mode. Once the output voltage exceeds in applications where SHDN may be driven above VIN, the input by 0.24V, the IC powers itself from VOUT instead of a resistor divider or other means must be employed to VIN. At this point the internal circuitry has no dependency keep the SHDN voltage below (VIN + 0.4V) to prevent the on the VIN input voltage, eliminating the requirement for possibility of the test mode being engaged. Please refer a large input capacitor. The input voltage can drop as low to Figure 1 for two possible implementations as 0.5V. The limiting factor for the application becomes the availability of the power source to supply sufficient LTC3528/LTC3528B LTC3528/LTC3528B power to the output at the low voltages, and the maximum VIN 4M 4M duty cycle, which is clamped at 93% typical. Note that ±30% SHDN ±30% SHDN at low input voltages, small voltage drops due to series 3528 F01 R resistance become critical, and greatly limit the power VCNTRL delivery capability of the converter. 1M ZETEX ZC2811E 1M R > (VCNTRL/(VIN + 0.4) – 1) MΩ VCNTRL
Figure 1. Recommended Shutdown Circuits when Driving SHDN Above VIN
3528fd 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts