Datasheet LTC3528-2 (Analog Devices) - 9

LTC3528-2
OPERATION (Refer to Block Diagram)
The LTC3528-2 is a 2MHz synchronous boost converter
LOW NOISE FIXED FREQUENCY OPERATION
housed in an 8-lead 3mm × 2mm DFN package. With the ability to start up and operate from inputs less than 0.88V,
Soft-Start
the device features fixed frequency, current mode PWM The LTC3528-2 contains internal circuitry to provide soft- control for exceptional line and load regulation. The cur- start operation. The internal soft-start circuitry slowly rent mode architecture with adaptive slope compensation ramps the peak inductor current from zero to its peak provides excellent transient load response and requires value of 1.5A (typical), allowing start-up into heavy loads. minimal output filtering. Internal soft-start and internal The soft-start time is approximately 0.5ms. The soft-start loop compensation simplifies the design process while circuitry is reset in the event of a commanded shutdown minimizing the number of external components. or a thermal shutdown. With its low RDS(ON) and low gate charge internal N-channel MOSFET switch and P-channel MOSFET synchronous recti-
Oscillator
fier, the LTC3528-2 achieves high efficiency over a wide An internal oscillator sets the frequency of operation to range of load current. The LTC3528-2 features continuous 2MHz. 2MHz PWM operation over a wide range of load current. At very light loads, the LTC3528-2 will enter Burst Mode
Shutdown
operation to maintain high efficiency. Operation can be The converter is shut down by pulling the SHDN pin below best understood by referring to the Block Diagram. 0.25V, and activated by pulling SHDN above 0.88V. Al- though SHDN can be driven above VIN or VOUT (up to the
LOW VOLTAGE START-UP
absolute maximum rating) without damage, the LTC3528- The LTC3528-2 includes an independent start-up oscillator 2 has a proprietary test mode that may be engaged if designed to operate at an input voltage of 0.70V (typical). SHDN is held in the range of 0.5V to 1V higher than the Soft-start and inrush current limiting are provided during greater of VIN or VOUT. If the test mode is engaged, normal start-up, as well as normal operating mode. PWM switching action is interrupted, which can cause undesirable operation in some applications. Therefore, When either VIN or VOUT exceeds 1.6V typical, the IC enters in applications where SHDN may be driven above V normal operating mode. Once the output voltage exceeds IN, a resistor divider or other means must be employed to the input by 0.24V, the IC powers itself from VOUT instead keep the SHDN voltage below (V of V IN + 0.4V) to prevent the IN. At this point the internal circuitry has no dependency possibility of the test mode being engaged. Please refer on the VIN input voltage, eliminating the requirement for to Figure 1 for two possible implementations a large input capacitor. The input voltage can drop as low as 0.5V. The limiting factor for the application becomes the availability of the power source to supply sufficient LTC3528-2 LTC3528-2 V power to the output at the low voltages, and the maximum IN 4M 4M ±30% ±30% duty cycle, which is clamped at 92% typical. Note that SHDN SHDN 3528 F01 at low input voltages, small voltage drops due to series R V resistance become critical, and greatly limit the power CNTRL 1M delivery capability of the converter. ZETEX ZC2811E 1M R > (VCNTRL/(VIN + 0.4) – 1) MΩ VCNTRL
Figure 1. Recommended Shutdown Circuits when Driving SHDN Above VIN
35282fb 9 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