Datasheet LTC3637 (Analog Devices) - 9

LTC3637
OPERATION (Refer to Block Diagram)
The LTC3637 is a step-down DC/DC regulator with an reducing the VIN pin supply current to only 12µA. As the internal high side power switch that uses Burst Mode load current discharges the output capacitor, the voltage control. The low quiescent current and high switching on the VFB pin decreases. When this voltage falls 5mV frequency results in high efficiency across a wide range below the 800mV reference, the feedback comparator of load currents. Burst Mode operation functions by us- trips and enables burst cycles. ing short “burst” cycles to switch the inductor current At the beginning of the burst cycle, the internal high side through the internal power MOSFET, followed by a sleep power switch (P-channel MOSFET) is turned on and the cycle where the power switch is off and the load current inductor current begins to ramp up. The inductor current is supplied by the output capacitor. During the sleep cycle, increases until either the current exceeds the peak current the LTC3637 draws only 12µA of supply current. At light comparator threshold or the voltage on the V loads, the burst cycles are a small percentage of the total FB pin exceeds 800mV, at which time the high side power switch is turned cycle time which minimizes the average supply current, off and the external catch diode turns on. The inductor greatly improving efficiency. Figure 1 shows an example current ramps down until the reverse current compara- of Burst Mode operation. The switching frequency and the tor trips, signaling that the current is close to zero. If the number of switching cycles during Burst Mode operation voltage on the V are dependent on the inductor value, peak current, load FB pin is still less than the 800mV refer- ence, the high side power switch is turned on again and current, input voltage and output voltage. another cycle commences. The average current during a SLEEP burst cycle will normally be greater than the average load CYCLE SWITCHING BURST FREQUENCY current. For this architecture, the maximum average output CYCLE current is equal to half of the peak current. The hysteretic nature of this control architecture results INDUCTOR CURRENT in a switching frequency that is a function of the input voltage, output voltage, and inductor value. This behavior BURST FREQUENCY provides inherent short-circuit protection. If the output is shorted to ground, the inductor current will decay very OUTPUT slowly during a single switching cycle. Since the high side VOLTAGE switch turns on only when the inductor current is near ∆VOUT 3637 F01 zero, the LTC3637 inherently switches at a lower frequency during start-up or short-circuit conditions.
Figure 1. Burst Mode Operation Start-Up and Shutdown Main Control Loop
If the voltage on the RUN pin is less than 0.7V, the LTC3637 The LTC3637 uses the VPRG1 and VPRG2 control pins to enters a shutdown mode in which all internal circuitry is connect internal feedback resistors to the VFB pin. This disabled, reducing the DC supply current to 3µA. When the enables fixed outputs of 1.8V, 3.3V or 5V without increas- voltage on the RUN pin exceeds 1.21V, normal operation ing component count, input supply current or exposure to of the main control loop is enabled. The RUN pin com- noise on the sensitive input to the feedback comparator. parator has 110mV of internal hysteresis, and therefore External feedback resistors (adjustable mode) can still must fall below 1.1V to stop switching and disable the be used by connecting both VPRG1 and VPRG2 to ground. main control loop. In adjustable mode the feedback comparator monitors An internal 0.8ms soft-start function limits the ramp rate the voltage on the VFB pin and compares it to an inter- of the output voltage on start-up to prevent excessive input nal 800mV reference. If this voltage is greater than the supply droop. If a longer ramp time and consequently less reference, the comparator activates a sleep mode in which supply droop is desired, a capacitor can be placed from the power switch and current comparators are disabled, 3637fa For more information www.linear.com/LTC3637 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 Typical Application Related Parts