Datasheet LTC3526L, LTC3526LB (Analog Devices) - 8

LTC3526L/LTC3526LB operAtion
(Refer to Block Diagram)
exceeds the input by 0.24V, the IC powers itself from LTC3526L/LTC3526LB LTC3526L/LTC3526LB VOUT instead of VIN. At this point the internal circuitry has VIN no dependency on the V 4M 4M IN input voltage, eliminating the ±30% ±30% requirement for a large input capacitor. The input voltage SHDN SHDN can drop as low as 0.5V. The limiting factor for the ap- VCNTRL plication becomes the availability of the power source to R 1M ZETEX ZC2811E supply sufficient energy to the output at low voltages, and VCNTRL 1M maximum duty cycle, which is clamped at 90% typical. R > (VCNTRL/(VIN + 0.4) – 1)MΩ Note that at low input voltages, small voltage drops due 3526l F01 to series resistance become critical, and greatly limit the
Figure 1. Recommended Shutdown Circuits when Driving SHDN above V
power delivery capability of the converter.
IN Error Amplifier LOW NOISE FIXED FREQUENCY OPERATION
The positive input of the transconductance error amplifier is internally connected to the 1.195V reference and the
Soft-Start
negative input is connected to FB. Clamps limit the mini- The LTC3526L/LTC3526LB contain internal circuitry to mum and maximum error amp output voltage for improved provide soft-start operation. The soft-start circuitry slowly large-signal transient response. Power converter control ramps the peak inductor current from zero to its peak value loop compensation is provided internally. An external of 750mA (typical) in approximately 0.5ms, allowing start- resistive voltage divider from VOUT to ground programs up into heavy loads. The soft-start circuitry is reset in the the output voltage via FB from 1.5V to 5.25V. event of a shutdown command or a thermal shutdown.  R2 V 1 195 . V  OUT = • 1+
Oscillator
 R1 An internal oscillator sets the switching frequency to 1MHz.
Current Sensing
Lossless current sensing converts the peak current signal of
Shutdown
the N-channel MOSFET switch into a voltage that is summed Shutdown is accomplished by pul ing the SHDN pin with the internal slope compensation. The summed signal below 0.3V and enabled by pulling the SHDN pin above is compared to the error amplifier output to provide a peak 0.8V. Although SHDN can be driven above V current control command for the PWM. IN or VOUT (up to the absolute maximum rating) without damage, the LTC3526L/LTC3526LB have a proprietary test mode
Current Limit
that may be engaged if SHDN is held in the range of 0.5V The current limit comparator shuts off the N-channel to 1V higher than the greater of VIN or VOUT. If the test MOSFET switch once its threshold is reached. The cur- mode is engaged, normal PWM switching action is inter- rent limit comparator delay to output is typically 60ns. rupted, which can cause undesirable operation in some Peak switch current is limited to approximately 750mA, applications. Therefore, in applications where SHDN may independent of input or output voltage, unless VOUT falls be driven above VIN, a resistor divider or other means below 0.7V, in which case the current limit is cut in half. must be employed to keep the SHDN voltage below (VIN + 0.4V) to prevent the possibility of the test mode being
Zero Current Comparator
engaged. Please refer to Figure 1 for two possible imple- The zero current comparator monitors the inductor cur- mentations. rent to the output and shuts off the synchronous rectifier 3526lfc 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 Typical Application Related Parts