LTC3526/LTC3526B function and working principle and component selection

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The internal structure of the LTC3526/LTC3526B and its external component connections in the application are shown below.

Both ICs have separate start-up oscillators that can be activated at an input voltage of 0.85V. The chip provides soft-start and inrush current limiting during startup and during normal operation. As long as the input voltage or output voltage exceeds 1.4V, the IC enters normal operation mode. When the output voltage is higher than the input voltage of 0.24V, the IC replaces VIN from the output self-power supply, so a smaller input capacitor can be selected. When VIN drops to 0.5V, the IC can still operate and the duty cycle is clamped at 90%. During soft start, the current of inductor L1 ramps from zero to 700 mA within 0.5 ms. In the off mode, the soft start circuit is reset. For 60ns, the peak switch current is limited to 700mA. The zero current comparator in the IC monitors the inductor current. When the current is reduced to approximately 30 mA, the synchronous rectifier is turned off to improve efficiency at light loads.

The anti-ringing circuit is connected to a resistor across the external inductor, which acts to block high frequency ringing on the pin SW and generate EMI radiation. The LTC3526/LTC3526B allow the actual output to be disconnected. During the IC off period, VOUT can be changed to OV, no current is drawn from the input, and VOUT is allowed to rise, and no reverse current flows into the battery on the input.

The LTC3526 automatically enters burst mode operation at light loads. When the load current increases, the IC reverts to the fixed frequency PWM mode. The LTC3526B features operation at a frequency of 1MHz, and under very light loads, the LTC3526B will disable pulse skipping.

Even with VIN>VOUT, the LTC3526/LTC3526B maintain voltage regulation, but with low efficiency and reduced maximum output current. When the voltage on pin SHDN is lower than 0.3V, the IC will turn off; if the voltage on SHDN is higher than 0.8V, lC will be enabled. If the die junction temperature exceeds 160 ° C, the IC enters the shutdown mode; as long as the die temperature drops below 145C, the IC will re-enable. can.

Ll selects a 3.3-6.8μH chip inductor and requires a low equivalent series resistance (ESR). The allowable inductor current ripple (peak-to-peak) is typically 30% of the maximum inductor current, and the minimum inductor value is determined by the following equation:

In the application circuit of the LTC3526/LTC3526B, the input capacitor CIN can be a low-ESR 1μF ceramic capacitor, and the output capacitor COUT is a 4.7-10μF low-ESR multilayer ceramic capacitor.

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