Datasheet LTC3526L-2, LTC3526LB-2 (Analog Devices) - 8

LTC3526L-2/LTC3526LB-2 operaTion
(Refer to Block Diagram)
exceeds the input by 0.24V, the IC powers itself from LTC3526L-2/LTC3526LB-2 LTC3526L-2/LTC3526LB-2 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 maximum duty cycle, which is clamped at 90% typical. 1M R > (V Note that at low input voltages, small voltage drops due CNTRL/(VIN + 0.4) – 1)MΩ 3526lb2 F01 to series resistance become critical, and greatly limit the
Figure 1. Recommended Shutdown Circuits when Driving
power delivery capability of the converter.
SHDN above VIN Error Amplifier LOW NOISE FIXED FREQUENCY OPERATION
The positive input of the transconductance error amplifier
Soft-Start
is internally connected to the 1.195V reference and the negative input is connected to FB. Clamps limit the mini- The LTC3526L-2/LTC3526LB-2 contain internal circuitry mum and maximum error amp output voltage for improved to provide soft-start operation. The soft-start circuitry large-signal transient response. Power converter control slowly ramps the peak inductor current from zero to its loop compensation is provided internally. An external peak value of 750mA (typical) in approximately 0.5ms, resistive voltage divider from VOUT to ground programs allowing start-up into heavy loads. The soft-start circuitry the output voltage via FB from 1.5V to 5.25V. is reset in the event of a shutdown command or a thermal shutdown.  R2 V 1. V  OUT = 195 • + 1  R1
Oscillator
An internal oscillator sets the switching frequency to
Current Sensing
2MHz. Lossless current sensing converts the peak current signal of the N-channel MOSFET switch into a voltage that is summed
Shutdown
with the internal slope compensation. The summed signal Shutdown is accomplished by pul ing the SHDN pin is compared to the error amplifier output to provide a peak below 0.3V and enabled by pulling the SHDN pin above current control command for the PWM. 0.8V. Although SHDN can be driven above VIN or VOUT (up to the absolute maximum rating) without damage,
Current Limit
the LTC3526L-2/LTC3526LB-2 have a proprietary test The current limit comparator shuts off the N-channel mode that may be engaged if SHDN is held in the range MOSFET switch once its threshold is reached. The cur- of 0.5V to 1V higher than the greater of VIN or VOUT. If rent limit comparator delay to output is typically 60ns. the test mode is engaged, normal PWM switching action Peak switch current is limited to approximately 750mA, is interrupted, which can cause undesirable operation independent of input or output voltage, unless VOUT falls in some applications. Therefore, in applications where below 0.7V, in which case the current limit is cut in half. SHDN may be driven above VIN, a resistor divider or other means must be employed to keep the SHDN voltage below (VIN + 0.4V) to prevent the possibility of the test mode being engaged. Please refer to Figure 1 for two possible implementations. 3526lb2fa 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