* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download VIN_DPM app note _ mod
Immunity-aware programming wikipedia , lookup
Analog-to-digital converter wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Josephson voltage standard wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Integrating ADC wikipedia , lookup
Wilson current mirror wikipedia , lookup
Power electronics wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Power MOSFET wikipedia , lookup
Current source wikipedia , lookup
Surge protector wikipedia , lookup
Operational amplifier wikipedia , lookup
Schmitt trigger wikipedia , lookup
Voltage regulator wikipedia , lookup
Battery charger wikipedia , lookup
Current mirror wikipedia , lookup
Opto-isolator wikipedia , lookup
Introduction: Designers of rechargeable battery powered equipment want a charger that minimizes charge time with maximum charge current by maximizing the power taken from the supply without collapsing the supply. Resistances between the supply and the battery present a challenge. This app note explains how to design the charging circuit to achieve the maximum power from the adapter despite the resistances between the supply and battery. 1) General working of switch-mode charger: Below is a circuit model of the buck converter based charger including all of the undesired resistances, including the inductor’s dc resistance (DCR) CHARGER BUCK POWER STAGE VBUS PIN SW VSUPPLY RIN = RWIRE + RCONNECTOR + RTRACE SYS L RBFET RIND HSFET LSFET BATFET BAT PIN ROUT = RWIRE + RTRACE + RCONNECTOR Figure 1: Charger model with resistances The input voltage to the charger is through the VBUS, since it is assumed to be from VSUPPLY which is typically a USB port or adapter. We will use this model to derive the minimum VSUPPLY voltage given the battery regulation threshold. Brief working of the charger: The charger works in 3 main modes of operation, depending on the battery voltage. 1) Low battery voltage signifies deeply discharged battery, hence it must be charged by a low value of current, till it is brought to the threshold value of VLOWV. This is known as Pre-Charge mode. 2) Once the battery voltage increases to a certain threshold (VLOWV), the full charge current which has been set is allowed to flow. This current is maintained by a regulation loop known as the Current Regulation / Constant Current Mode. 3) When the Battery voltage increases to VBATREG, it indicates that it is fully charged. Hence, charge current is tapered down. Charger now operates in Voltage Regulation / Constant Voltage Mode. VBATREG is typically 4.2V for Li-Ion batteries. The charger ideally must be able to allow the full value of charge current that it has been set for, till VBAT=4.2V. More importantly, it must be able to let the battery charge to VBATREG. Figure 2: Charge profile for the BQ2425x charger To determine the minimum VBUS value permissible, the designer needs to watch for the following issues: 1. Operation headroom between VBUS and VBAT to reach target charge current 2. Switching regulator maximum duty cycle Operation Headroom The resistance in MOSFETs and inductors generates voltage drop as current flows. If the voltage difference between VBUS and VBAT is too small, the target charge current cannot be achieved. For example, if VBUS is 4.3V, VBAT is 4.2V and total resistance from VBUS to VBAT is 150m, the maximum current between VBUS and VBAT is 660mA. Switching Regulator Maximum Duty Cycle Realistically, no high-side NMOS buck converter can reach 100% duty cycle. There is always dead time to avoid shoot-through during HSFET/LSFET turn-on/turn-off. If the duty cycle exceeds the maximum value, the switching regulator will skip some LSFET turn-on pulses to maintain average output current/voltage. Calculating minimum VBUS threshold The VBUS-MIN threshold is the minimum VBUS voltage to support target charge current and keep the duty cycle below the maximum duty cycle. We can derive the VBUS voltage through inductor ripple current calculation. IRIPPLE ICHG Inductor Current SW D 1-D Time Figure 3: Inductor current vs operation duty cycle I RIPPLE V T L 1 On the inductor current rising edge, I RIPPLE (VBUS I CHG ( RRBFET RHSFET ) I CHG ( RIND RBATFET ) VBAT ) D L 2 On the inductor current falling edge, I RIPPLE (VBAT I CHG ( RIND RBATFET ) I CHG RLSFET ) (1 D) L 3 Since the ripple current is the same, we have the following VBUS equation derived: VBUS 1 1 D VBAT I CHG ( RRBFET RHSFET RIND RBATFET ) I CHG ( RBATFET RIND RLSFET ) D D 4 We have a few assumptions to simplify the equation above. With 2.2uH, the ripple current at 96% duty cycle is less than 300mA. I is considered as average current. With maximum duty cycle of 96%, 1 D is only 4.2% compared to the second D item in the equation. Therefore, the third item can be ignored. Therefore the VBUS-MIN threshold is listed as the VBUS voltage at maximum duty cycle. 5 If the Input (VBUS) to the charger falls below the above calculated threshold, then the battery will not fully charge. 2) Minimum Supply Voltage: This section shows how the Input voltage to the charger can fall below the permissible value, when USB adapters are used due to input line resistance. The USB specification states that the output to the device from a low power port can be as low as 4.1V under full load after passing through all hubs and cables. Below is a circuit model of a single cell battery charger powered from USB port. Assume the 5V supply is an ideal voltage source with zero resistance in series, and the R USB is the total resistance of the cable, connector and PCB trance. The charger is modeled as an ideal buck converter which can reach 100% duty cycle. VBUS 5V Supply (i.e. USB port) RIN Charger IC RIN = RWIRE + RCONNECTOR + RTRACE The input voltage VBUS at the charger has to be above the battery charging regulation threshold VBAT_REG (typically 4.2V). Assume that the minimum to which VIN falls is VINMIN = 4.75V. VBUS=VINMIN - IINMAX x RIN > VBAT_REG With USB cable resistance of 400m, table below shows the minimum VBUS voltage from USB2.0 port and USB 1.5A adapter. VINMIN IINMAX VBUS Below VBUS-MIN? USB 2.0 4.75V 0.5A 4.55V No USB 1.5A Adapter 4.75V 1.5A 4.15V Yes At maximum duty cycle the VBUS voltage is close to VBAT so IIN ~= ICHG. Equation 5 can now be expanded to determine the minimum input supply voltage for a given charge current. VSUPPLYMIN VBAT I CHG ( RIN RRBFET RHSFET RIND RBATFET ) DMAX 6 The designer can use equation 6 to determine how low of resistance (high of quality) of cable and connector or how wide/thick of PCB trace is necessary to avoid excessive voltage drop at the VBUS pin and therefore utilize all of the adapter’s power for charging the battery. 3) Input Voltage based Dynamic Power Management (VIN-DPM) If multiple adapters and/or cables and/or connectors are expected to be used, it may be difficult to design for all line resistance scenarios. A charger with VINDPM prevents the input voltage from crashing regardless of input line resistance. What is VIN-DPM? VIN-DPM is an analog loop included in many of TI’s chargers. The purpose of the loop is to extract the maximum amount of current available from the supply without crashing the adapter, i.e. the input current (and therefore the resulting charge current) is limited in order to maintain supply voltage at VIN-DPM. This feature can be used when a USB port is one of the input power sources. Necessity for VINDPM: Consider using a charger without the VIN-DPM protection. As the load current drawn increases, the input current also increases, and hence the drop across the supply resistance increases. The voltage seen at the input pin of the charger is less than the EMF offered by the supply. Also, the supply (voltage source) has a compliance limit on the amount of current it can produce. Now when a load current is drawn such that the input current required, to maintain the sum of charge current and load current, is beyond the capability of the supply, the input voltage starts to fall (input capacitor discharges as an attempt to supply the high current demanded). When this hits the Under-voltage threshold, the charger turns off. Now, this allows the input voltage to recover (as the input capacitor recharges), and once it rises above the UVLO, charging again begins. Once the charger turns on, the same cycle repeats. This causes an On/Off pulsing in the charge current as shown in the figure below: VIN UVLO 1V/div IIN 500mA / div Now, if a charger with VIN-DPM was used, it prevents the non-ideal pulsing of charge current by limiting input current. It works as follows: as the input voltage reduces and hits the set VINDPM threshold, the VINDPM function activates to limit the input current to a smaller value. This prevents the input voltage from crashing to the Under Voltage point, hence ensuring that ICHG pulsing does not occur. At present, adapters supply currents between 100mA to several amperes. With VINDPM, there is no need to design charger IC with some specific adapter in mind. For example, if it was used without VIN-DPM, and IC was programmed to draw an input of 900mA, it would cause an adapter of 500mA capability to crash. Conclusion: Resistances between the supply and charger can prevent the charger from pulling the maximum power from its supply adapter without collapsing the adapter voltage. An equation for determining the minimum supply voltage required for a charger to provide the maximum charge current from given supply adapter without collapsing the adapter voltage was developed. In addition, the VINDPM feature allows the use of a variety of adapters and/or power connections without fear of collapsing the adapter voltage because it limits the charger’s input current.