Buck-boost type controller extends the working time of the handheld device

　　Author ：Iflowpower – Portable Power Station Supplier

Background With small, high-energy density modern battery technology, high-efficiency, high-efficiency, high-energy, high-energy density, high-efficiency. This gives a design challenge to a 3.3V bus voltage, a system powered by a lithium ion and lithium polymer battery.

In particular, the 3.3V bus voltage is supplied with a load current greater than 0.5A, even.

Although the buck converter is specially converted to a lower output voltage (eg 1.8V) with a lithium-ion battery voltage of 2.7-4.

2V, the boost converter can highly efficiently output voltage (eg 5V), but both converters do not always appear 3.3V bus voltage. Single-ended main Inductor Converter (Sepic), a topology such as a boost and a buck converter can utilize all battery power, but it is low efficient, high cost, deposit area, and more devices and complex Higher disadvantages.

Single or two lithium ion batteries typically charge with 5 to 9V AC adapter. Ability to replace the battery directly with the AC adapter, shorten battery charging time. Of course, this requires that the converter can not only work at the lowest battery input voltage, but also work with an elevated input voltage.

Traditionally, the handheld device powered by the battery directly draws power from the battery, even in the battery charging. This type of DC / DC converter also has a very low quiescent current to save battery energy in standby or idle mode. Synchronous buck-boosting controller is an ideal solution to supply 3.

3V power rails in hand-held devices that are powered by a single-cell lithium-ion battery. 3.3V power rail is a complex problem.

Many companies have supplied solutions. Linglurt's LTC3785 Synchronous buck-boost type switching regulator DC / DC controller is one of them. Its proprietary buck-boost topology only has a single inductor, which can use a fixed output voltage higher than, equal to or lower than the input voltage of the output voltage.

LTC3785 works with input and output voltages of 2.7 to 10V, which is ideal for single or two lithium ions or lithium polymer batteries, or multi-alkaline nickel-hydrogen metal, nickel cadmium or alkaline batteries. This highly integrated controller uses little fiberly, with many programmable functions, such as soft boot, switching frequency, and limit threshold voltage.

The efficiency curve is in the synchronization, 4 switch buffet-boost-boost-boost DC / DC converter, with 2.7 ~ 10V input voltage, 3.3V / 3A fixation in efficiency of up to 96% Output.

The LTC3785 supplies all N-channel MOSFET gate drivers to supply convenience for using low RDS (ON) single package multi-power switch technology. Its proprietary topology and control architecture uses MOSFET RDS to detect forward and reverse current limiting, achieving unparalleled high efficiency. When you want to increase the current limiting accuracy, you can use the detection resistor.

In addition, the LTC3785 can work in burstmode, down the static current of the light load to less than 100μA, which is critical to extending the battery operating time of portable applications. The LTC3785 also has a real output disconnection capability while downtime, ensuring the disconnection of the battery and system load. Figure 1 Step-pressure-boosting converter schematic and its efficiency curve advanced control topology maximum efficiency LTC3785 based on standard H bridge buck-boost power level, as shown in Figure 2.

It also contains buck and boost switch MOSFET, which connects to a single inductor. Unlocking all 4 MOSFETs with continuous simultaneous switch - Different boost mode, LTC3785 is designed, only two MOSFETs each time. Many buck-boost control circuits have efficiently decline in conversion points, power jitter or output voltage unstable problems.

However, the LTC3785 seamlessly conversion between step-down, buck-boost and boosting workspace, maintains low noise performance in all operating modes. This control circuit also greatly reduces unnecessary switches and conduction loss, maximizing the efficiency of converters. Figure 2 The power supply partial block diagram and the working mode operating mode input voltage is higher than the output voltage, the converter operates, the switch A and B conversion input voltage, the switch D is turned on, the L1 is connected to the output (see Figure 2).

As the input voltage is reduced and proximate the output voltage, the converter is close to the maximum duty cycle of the buck mode, the boost portion of the bridge begins the switch, into the buck-boost or 4 switching work area. As the input voltage is further reduced, the converter enters the boost area. Switch A is turned on at the minimum boost duty cycle, the inductor is connected to the input, switches C, and D conversion of the output side of the inductor between the output capacitor and the ground, as the synchronous boost converter.

Other functions to achieve higher flexibility LTC3785 There are other functions to improve their availability in portable applications, for example, extremely low static current to extend battery working hours. For such portable applications, the device can be configured to work in burst mode to extend battery working hours. In the burst mode, the LTC 3785 supplies energy to the output until the output voltage reaches a steady state.

After reaching the steady state, the device is placed in a sleep state, and the driving of the external MOSFET is turned off, and only the critical circuit is maintained, and the LTC3785 consumes less than 100μA current. The load current is supplied by the output capacitor during this period. When the output voltage drops below lower voltage regulatory edge, the device "wakes up" and starts the switching operation again, and the output capacitor is charged again.

LTC3785 also supplies overload and short circuit protection by detecting and limiting the input current from the input power supply. If the user programming limit value is reached, the soft start capacitor connected to the RUN / SS pin is again used to use the fault press and start discharge. If the current limiting state continues for a long time, the converter will be disabled while starting the reset press to restart the converter.

If the LTC3785 cannot restart, and the overload status continues, this working mode will continue to limit the total power consumption. By supplying a small current to the RUN / SS pin, you can also lock the device lock instead of automatic restart. Due to the change in external MOSFET resistance, MOSFET leak detection is generally not very accurate.

If you want more stringent current limiting accuracy, you can add current detection resistance. LTC3785 can also be programmed to achieve full-level work to allow converters to supply and absorb currents equal to current flow setting points. This is achieved by determining the high logic level signal on the CCM pin.

Internal P-channel low pressure difference regulator with input power supply voltage in VCC pin 4.35V voltage. This voltage is powered by the drive and the LTC 3785 internal circuit, which can supply 100 mA peak current, which must be bypass the capacitor with a minimum of 4.

7μF. The VCC regulator can be connected to Vout by a Schottky diode to supply higher gate drive currents. Finally, the LTC3785 contains overvoltage and undervoltage functions for failure protection and transient restrictions.

If the output voltage is detected by 9.5% higher than the target voltage regulation point, then the switch action stops. The output voltage will then be reduced to a safer level because there is no energy supply to the output.

Once the output is sufficiently declining, the switching action will start again. In the case of overvoltage, the integrated circuit is forcibly working in a fixed frequency mode, and the burst mode is prohibited. N channel power MOSFET selection and solution size LTC3785 to 4 external N-channel power MOSFETs, two for top switches, and two other signs.

The important parameters are VBR (DSS), VGS (TH), RDS (ON) and IDS (MAX). Drive voltage 4.35VVCC power setting.

For most input voltages, low logic gate thresholds MOSFETs can be used for applications where most input voltages are less than 5V. In addition, typical LTC3785DC / DC converters all use ceramic input and output capacitors. A 10W output full circuit deposit area is less than 2cm2cm, the inductor is the highest device, the height is 0.

32cm. .