L6926 STMICROELECTRONICS | Alldatasheet
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Technical content
1 FEATURES
2 APPLICATIONS
3 DESCRIPTION
designed to provide extremely high efficiency. Thermal Shutdown (150°C) are also present. Figure 2. Application Test Circuit Figure 1. Packages Table 1. Order Codes
Table 2. Absolute Maximum Ratings Figure 3. Pin Connection Table 3. Thermal Data Table 4. Pin Functions 1 RUN Shutdown input. When connected to a low leve l (lower than 0.4V) the device stops working. When high (higher than 1.3V) the device is enabled. 220pF capacitor is enough to guarantee the loop stability. voltage by connecting this pin to an external resistor divider. 5 LX Switch output node. This pin is internally connected to the drain of the internal switches. from 2V to 5.5V. An internal UVLO circuit realizes a 100mV (typ.) hysteresis.
Table 5. Electrical Characteristics (Tj = 25°C, VCC = 3.6V unless otherwise specified) synchronization circuit is activated and the device works at the same switching frequency.
4 OPERATION DESCRIPTION
Figure 4. Device Block Diagram
4.1 Modes of Operation
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 dis- charges 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 600KHz. 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 500KHz up to 1.4MHz. 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).
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: 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 TOFF 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: Where IPEAK is the valley current limit (1.4A typ.) and TON_MIN is the minimum TON of the high side MOSFET.
4.3 Slope Compensation
∆ION V IN V OUT–() ∆IOFF V OUT() IPEAK IVALLEY V IN
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 sta- bility. 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.
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:
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 V OUT 0.6 1 R 2 R 1 ⋅=
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.
Figure 5. MSOP8 Mechanical Data & Package Dimensions Note: 1. D and F does not include mold flash or protrusions.
Figure 6. VFSON8 Mechanical Data & Package Dimensions
Table 6. Revision History January 2004 2 First Issue in EDOCS dms. September 2004 3 Changed the style look & feel. Add. V8 and V7 parameter in the Table 2 - Absolute Maximum Ratings.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners © 2004 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America L6926