Datasheet LTC7138 (Analog Devices) - 8

LTC7138
operaTion (Refer to Block Diagram)
The LTC7138 is a step-down DC/DC regulator with internal External feedback resistors (adjustable mode) can be used power switch that uses Burst Mode control, combining by connecting both VPRG1 and VPRG2 to ground. low quiescent current with high switching frequency, In adjustable mode the feedback comparator monitors which results in high efficiency across a wide range of the voltage on the V load currents. Burst Mode operation functions by us- FB pin and compares it to an internal 800mV reference. If this voltage is greater than the refer- ing short “burst” cycles to switch the inductor current ence, the comparator activates a sleep mode in which through the internal power MOSFET, followed by a sleep the power switch and current comparators are disabled, cycle where the power switch is off and the load current reducing the V is supplied by the output capacitor. During the sleep cycle, IN pin supply current to only 12µA. As the load current discharges the output capacitor, the voltage the LTC7138 draws only 12µA of supply current. At light on the V loads, the burst cycles are a small percentage of the total FB pin decreases. When this voltage falls 5mV below the 800mV reference, the feedback comparator cycle time which minimizes the average supply current, trips and enables burst cycles. greatly improving efficiency. Figure 1 shows an example of Burst Mode operation. The switching frequency is de- At the beginning of the burst cycle, the internal high side pendent on the inductor value, peak current, input voltage power switch (P-channel MOSFET) is turned on and the and output voltage. inductor current begins to ramp up. The inductor current increases until either the current exceeds the peak cur- SLEEP rent comparator threshold or the voltage on the VFB pin CYCLE SWITCHING BURST FREQUENCY exceeds 800mV, at which time the switch is turned off CYCLE and the inductor current is carried by the external catch diode. The inductor current, then sensed through the INDUCTOR CURRENT ANODE pin, ramps down until the current falls below the valley current comparator threshold. If the voltage on the BURST FREQUENCY VFB pin is still less than the 800mV reference, the power switch is turned on again and another cycle commences. OUTPUT The average current during a burst cycle will normally be VOLTAGE greater than the average load current. For this architecture, ∆VOUT 7138 F01 the maximum average output current is equal to 75% of the peak current.
Figure 1. Burst Mode Operation
The hysteretic nature of this control architecture results in a switching frequency that is a function of the input
Main Control Loop
voltage, output voltage, and inductor value. This behavior The LTC7138 uses the V provides inherent short-circuit protection. If the output is PRG1 and VPRG2 control pins to connect internal feedback resistors to the V shorted to ground, the inductor current will decay very FB pin. This enables fixed outputs of 1.8V, 3.3V or 5V without increas- slowly during a single switching cycle. Since the high side ing component count, input supply current or exposure to switch turns on only when the inductor current is below noise on the sensitive input to the feedback comparator. the valley current trip threshold, the LTC7138 inherently switches at a lower frequency during start-up or short- circuit conditions. 7138f 8 For more information www.linear.com/LTC7138 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