Datasheet LTC3499, LTC3499B (Analog Devices) - 9

LTC3499/LTC3499B
OPERATION Reverse-Battery Protection
capacitor will recharge causing the LTC3499 to re-enter Connecting the battery backwards poses a severe problem sleep if the output load current remains less than the to most power converters. At a minimum the battery will sleep threshold. The frequency of this intermittent PWM be quickly discharged. Almost all ICs have an inherent diode (or burst) operation is proportional to load current. from V Therefore, as the load current drops further below the IN (cathode) to ground (anode) which conducts ap- preciable current when V burst threshold, the LTC3499 operates in PWM mode IN drops more than 0.7V below ground. Under this condition the integrated circuit will less frequently. When the load current increases above most likely be damaged due to the excessive current draw. the burst threshold, the LC3499 will resume continuous There exists the possibility for the battery and circuitry PWM operation seamlessly. powered by the device to also be damaged. The LTC3499/ Referring to the Functional Block Diagram, an optional LTC3499B have integrated circuitry which allows negligible capacitor, CFF, between VOUT and FB in some circumstances current flow under a reverse-battery condition, protecting can reduce peak-to-peak VOUT ripple and input quiescent the battery, device and circuitry attached to the output. A current during Burst Mode operation. Typical values for graph of the reverse-battery current drawn is shown in CFF range from 10pF to 220pF. the Typical Performance Characteristics.
Output Disconnect and Inrush Current Limiting
Discrete methods of reverse battery protection put ad- ditional dissipative elements in the high current path The LTC3499/LTC3499B are designed to allow true output reducing efficiency while increasing component count disconnect by eliminating body diode conduction of the to implement protection. The LTC3499/LTC3499B do not internal P-channel MOSFET switch. This allows VOUT to suffer from either of these drawbacks. go to zero volts during shutdown without drawing any current from the input source. It also provides for inrush
Burst Mode Operation (LTC3499 only)
current limiting at turn-on, minimizing surge current seen Portable devices frequently spend extended time in low by the input supply. power or stand-by mode, only drawing high power when
V
specific functions are enabled. In order to improve battery
IN > VOUT Operation
life in these types of products, high power converter ef- The LTC3499/LTC3499B will maintain voltage regulation ficiency needs to be maintained over a wide output power when the input voltage is above the output voltage. This is range. In addition to its high efficiency at moderate and achieved by terminating the switching on the synchronous heavy loads, the LTC3499 includes automatic Burst Mode P-channel MOSFET and applying VIN statically on the gate. operation that improves efficiency of the power converter This will ensure the volts • seconds of the inductor will at light loads. Burst Mode operation is initiated if the reverse during the time current is flowing to the output. output load current falls below an internally programmed Since this mode will dissipate more power in the IC, the threshold (see Typical Performance graph, Output Load maximum output current is limited in order to maintain Burst Mode Threshold vs VIN). Once initiated the Burst an acceptable junction temperature: Mode operation circuitry shuts down most of the circuitry 125 – T in the LTC3499, keeping alive only the circuitry required I A OUT(MAX) ≅ to monitor the output voltage. ( ) θJA • VIN +1.5 – VOUT This state is referred to as sleep. In sleep, the LTC3499 where TA = ambient temperature and θJA is the package only draws 20µA from the input supply, greatly enhancing thermal resistance (45°C/W for the DD8 and 160°C/W efficiency. When the output has drooped approximately for the MS8). 1% from its nominal regulation point, the LTC3499 wakes For example at V up and commences normal PWM operation. The output IN = 4.5V, VOUT = 3.3V and TA = 85°C in the DD8 package, the maximum output current is 330mA. 3499fc 9