L6928 STMICROELECTRONICS | Alldatasheet
Document overview
- Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
- PDF pages: 16
Technical content
Datasheet sections
- 1 Pin settings
- 1.1 Pin connection
- 1.2 Pin description
- 2 Maximum ratings
- 2.1 Absolute maximum ratings
- 2.2 Thermal data
- 3 Electrical characteristics
- 4 Operation description
- 4.1 Modes of operation
- 4.1.1 Low consumption mode
- 4.1.2 Low noise mode
- 4.1.3 Synchronization
- 4.2 Short circuit protection
- 4.3 Slope compensation
- 4.4 Loop stability
- 5 Additional features and protections
- 5.1 DROPOUT operation
- 5.2 PGOOD (Power Good Output)
- 5.3 Adjustable output voltage
- 5.4 OVP (Overvoltage Protection)
- 5.5 Thermal shutdown
- 6 Package mechanical data
- 7 Order codes
- 8 Revision history
High efficiency monolithic synchronous step down regulator General features ■ 2V to 5.5V battery input range ■ High efficiency: up to 95% ■ Internal synchronous switch ■ No external schottky required ■ Extremely low quiescent current ■ 1µA max shutdown supply current ■ 800mA max output current ■ Adjustable output voltage from 0.6V ■ Low drop-out operation: up to 100% duty cycle ■ Selectable low noise/low consumption mode at light load ■ Power good signal ■ ±1% output voltage accuracy ■ Current-mode control ■ 1.4MHz switching frequency ■ Externally synchronizable from 1MHz to 2MHz ■ OVP ■ Short circuit protection
Applications
■ Battery-powered equipments ■ Portable instruments ■ Cellular phones ■ PDAs and hand held terminals ■ DSC ■ GPS
Description
The device is dc-dc monolithic regulator specifically designed to provide extremely high efficiency. L6928 supply voltage can be as low as 2V allowing its use in single Li-ion cell supplied applications. Output voltage can be selected by an external divider down to 0.6V. Duty Cycle can saturate to 100% allowing low drop-out operation. The device is based on a 1.4MHz fixed-frequency, current mode-architecture. Low Consumption Mode operation can be selected at light load conditions, allowing switching losses to be reduced. L6928 is externally synchronizable with a clock which makes it useful in noise-sensitive applications. Other features like Powergood, Overvoltage protection, Shortcircuit protection and Thermal Shutdown (150°C) are also present. MSOP8 VFQFPN8 Application test circuit L 4.7µH SYNC LX VFB 10µF 6.3V 220pF 10µF 6.3V 200K 500K 100K VIN=2V to 5.5V VOUT=1.8V VCC RUN COMP GND PGOOD D01IN1528
1 Pin settings
1.1 Pin connection
Figure 1. Pin connection (top view)
1.2 Pin description
Table 1. Pin description stops working. When high (higher than 1.3V) the device is enabled. 2C O M P Error amplifier output. A compensation network has to be connected to this pin. Usually a 220pF capacitor is enough to guarantee the loop stability. to the input voltage by connecting this pin to an external resistor divider.
7 SYNC
activated and the device works at the same switching frequency.
8 PGOOD
than 90% of the regulated output voltage. If not used the pin can be left floating.
2 Maximum ratings
2.1 Absolute maximum ratings
2.2 Thermal data
Table 2. Absolute maximum ratings Table 3. Thermal data
3 Electrical characteristics
Table 4. Electrical characteristcs
- Specification referred to T J from -40°C to +125°C. Specification over the -40 to +125°C TJ temperature range are assured
by design, characterization and statistical correlation. Table 4. Electrical characteristcs (continued)
4 Operation description
compensation signal and on the output of the error amplifier. meet the new load requirements. Figure 2. Device block diagram
Operation description L6928
4.1 Modes of operation
Depending on the SYNC pin value the device can operate in low consumption or low noise mode. If the SYNC pin is high (higher than 1.3V) the low consumption mode is selected while the low noise mode is selected if the SYNC pin is low (lower than 0.5V).
4.1.1 Low consumption mode
In this mode of operation, at light load, the device operates discontinuously based on the COMP pin voltage, in order to keep the efficiency very high also in these conditions. While the device is not switching the load discharges the output capacitor and the output voltage goes down. When the feedback voltage goes lower than the internal reference, the COMP pin voltage increases and when an internal threshold is reached, the device starts to switch. In these conditions the peak current limit is set approximately in the range of 200mA- 400mA, depending on the slope compensation (see related section). Once the device starts to switch the output capacitor is recharged. The feedback pin increases and, when it reaches a value slightly higher than the reference voltage, the output of the error amplifier goes down until a clamp is activated. At this point, the device stops to switch. In this phase, most of the internal circuitries are off, so reducing the device consumption down to a typical value of 25µA.
4.1.2 Low noise mode
If for noise reasons, the very low frequencies of the low consumption mode are undesirable, the low noise mode can be selected. In low noise mode, the efficiency is a little bit lower compared with the low consumption mode in very light load conditions but for medium-high load currents the efficiency values are very similar. Basically, the device switches with its internal free running frequency of 1.4MHz. Obviously, in very light load conditions, the device could skip some cycles in order to keep the output voltage in regulation.
4.1.3 Synchronization
The device can also be synchronized with an external signal from 1MHz up to 2MHz. In this case the low noise mode is automatically selected. The device will eventually skip some cycles in very light load conditions. The internal synchronization circuit is inhibited in shortcircuit and overvoltage conditions in order to keep the protections effective (see relative sections).
L6928 Operation description
4.2 Short circuit protection
During the device operation, the inductor current increases during the high side turn ON phase and decrease during the high side turn off phase based on the following equations: Equation 1 Equation 2 In strong overcurrent or shortcircuit conditions the VOUT can be very close to zero. In this case ∆ION increases and ∆IOFF decreases. When the inductor peak current reaches the current limit, the high side mosfet turns off and so the TON is reduced down to the minimum value (250ns typ.) in order to reduce as much as possible ∆ION. Anyway, if VOUT is low enough it can be that the inductor peak current further increases because during the TOFF the current decays very slowly. Due to this reason a second protection that fixes the maximum inductor valley current has been introduced. This protection doesn't allow the high side MOSFET to turn on if the current flowing through the inductor is higher that a specified threshold (valley current limit). Basically the T OFF is increased as much as required to bring the inductor current down to this threshold. So, the maximum peak current in worst case conditions will be: Equation 3 Where IPEAK is the valley current limit (1.4A typ.) and T ON_MIN is the minimum TON of the high side MOSFET. ∆ION VIN VOUT–() ∆IOFF VOUT() IPEAK IVALLEY VIN
Operation description L6928
4.3 Slope compensation
In current mode architectures, when the duty cycle of the application is higher than approximately 50%, a pulse-by-pulse instability (the so called sub harmonic oscillation) can occur. To allow loop stability also in these conditions a slope compensation is present. This is realized by reducing the current flowing through the inductor necessary to trigger the COMP comparator (with a fixed value for the COMP pin voltage). With a given duty cycle higher than 50%, the stability problem is particularly present with an higher input voltage (due to the increased current ripple across the inductor), so the slope compensation effect increases as the input voltage increases. From an application point of view, the final effect is that the peak current limit depends both on the duty cycle (if higher than approximately 40%) and on the input voltage.
4.4 Loop stability
Since the device is realized with a current mode architecture, the loop stability is usually not a big issue. For most of the application a 220pF connected between the COMP pin and ground is enough to guarantee the stability. In case very low ESR capacitors are used for the output filter, such as multilayer ceramic capacitors, the zero introduced by the capacitor itself can shift at very high frequency and the transient loop response could be affected. Adding a series resistor to the 220pF capacitor can solve this problem. The right value for the resistor (in the range of 50K) can be determined by checking the load transient response of the device. Basically, the output voltage has to be checked at the scope after the load steps required by the application. In case of stability problems, the output voltage could oscillates before to reach the regulated value after a load step.
L6928 Additional features and protections
5 Additional features and protections
5.1 DROPOUT operation
The Li-Ion battery voltage ranges from approximately 3V and 4.1V-4.2V (depending on the anode material). In case the regulated output voltage is from 2.5V and 3.3V, it can be that, close to the end of the battery life, the battery voltage goes down to the regulated one. In this case the device stops to switch, working at 100% of duty cycle, so minimizing the dropout voltage and the device losses.
5.2 PGOOD (Power Good Output)
A power good output signal is available. The VFB pin is internally connected to a comparator with a threshold set at 90% of the of reference voltage (0.6V). Since the output voltage is connected to the VFB pin by a resistor divider, when the output voltage goes lower than the regulated value, the VFB pin voltage goes lower than 90% of the internal reference value. The internal comparator is triggered and the PGOOD pin is pulled down. The pin is an open drain output and so, a pull up resistor should be connected to him. If the feature is not required, the pin can be left floating.
5.3 Adjustable output voltage
The output voltage can be adjusted by an external resistor divider from a minimum value of 0.6V up to the input voltage. The output voltage value is given by: Equation 4
5.4 OVP (Overvoltage Protection)
The device has an internal overvoltage protection circuit to protect the load. If the voltage at the feedback pin goes higher than an internal threshold set 10% (typ) higher than the reference voltage, the low side power MOSFET is turned on until the feedback voltage goes lower than the reference one. During the overvoltage circuit intervention, the zero crossing comparator is disabled so that the device is also able to sink current.
5.5 Thermal shutdown
The device has also a thermal shutdown protection activated when the junction temperature reaches 150°C. In this case both the high side MOSFET and the low side one are turned off. Once the junction temperature goes back lower than 95°C, the device restarts the normal operation. VOUT 0.6 1 R2 ⎛⎞⋅=
6 Package mechanical data
conditions are also marked on the inner box label. ECOPACK is an ST trademark. Figure 3. MSOP8 mechanical data & package dimensions Note: 1. D and F does not include mold flash or protrusions.
Figure 4. VFQFPN8 mechanical data & package dimensions
7 Order codes
Table 5. Order codes
8 Revision history
Table 6. Revision history Feb-2005 2 Changed from Product Preview to Final datasheet. Nov-2005 3 Updated Table 5. Electrical characteristics. Added VFQFPN8 package and new part numbers. 27-Oct-2006 4 Added RthJA for VFQFPN8 in Table 3. Document has been reformatted.