Datasheet LTC3526L-2, LTC3526LB-2 (Analog Devices) - 9

LTC3526L-2/LTC3526LB-2 operaTion
(Refer to Block Diagram) Zero Current Comparator Burst Mode OPERATION
The zero current comparator monitors the inductor cur- The LTC3526L-2 will enter Burst Mode operation at light rent to the output and shuts off the synchronous rectifier load current and return to fixed frequency PWM mode when this current reduces to approximately 30mA. This when the load increases. Refer to the Typical Performance prevents the inductor current from reversing in polarity, Characteristics to see the output load Burst Mode thresh- improving efficiency at light loads. old current vs VIN. The load current at which Burst Mode operation is entered can be changed by adjusting the
Synchronous Rectifier
inductor value. Raising the inductor value will lower the To control inrush current and to prevent the inductor load current at which Burst Mode operation is entered. current from running away when VOUT is close to VIN, the In Burst Mode operation, the LTC3526L-2 still switches at P-channel MOSFET synchronous rectifier is only enabled a fixed frequency of 2MHz, using the same error amplifier when VOUT > (VIN + 0.24V). and loop compensation for peak current mode control. This control method eliminates any output transient
Anti-Ringing Control
when switching between modes. In Burst Mode opera- The anti-ring circuit connects a resistor across the in- tion, energy is delivered to the output until it reaches the ductor to prevent high frequency ringing on the SW pin nominal regulation value, then the LTC3526L-2 transi- during discontinuous current mode operation. Although tions to sleep mode where the outputs are off and the the ringing of the resonant circuit formed by L and CSW LTC3526L-2 consumes only 9µA of quiescent current from (capacitance on SW pin) is low energy, it can cause EMI VOUT. When the output voltage droops slightly, switching radiation. resumes. This maximizes efficiency at very light loads by minimizing switching and quiescent losses. Burst Mode
Output Disconnect
output voltage ripple, which is typically 1% peak-to-peak, The LTC3526L-2/LTC3526LB-2 are designed to allow true can be reduced by using more output capacitance (10µF output disconnect by eliminating body diode conduction or greater), or with a small capacitor (10pF to 50pF) con- of the internal P-channel MOSFET rectifier. This allows for nected between VOUT and FB. VOUT to go to zero volts during shutdown, drawing no cur- As the load current increases, the LTC3526L-2 will au- rent from the input source. It also allows for inrush current tomatically leave Burst Mode operation. Note that larger limiting at turn-on, minimizing surge currents seen by the output capacitor values may cause this transition to oc- input supply. Note that to obtain the advantages of output cur at lighter loads. Once the LTC3526L-2 has left Burst disconnect, there must not be an external Schottky diode Mode operation and returned to normal operation, it will connected between SW and VOUT. The output disconnect remain there until the output load is reduced below the feature also allows VOUT to be pulled high, without any burst threshold current. reverse current into a battery connected to VIN. Burst Mode operation is inhibited during start-up and soft-
Thermal Shutdown
start and until VOUT is at least 0.24V greater than VIN. If the die temperature exceeds 160°C, the LTC3526L-2/ The LTC3526LB-2 features continuous PWM operation at LTC3526LB-2 will go into thermal shutdown. All switches 2MHz. At very light loads, the LTC3526LB-2 will exhibit will be off and the soft-start capacitor will be discharged. pulse-skipping operation. The device will be enabled again when the die temperature drops by about 15°C. 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