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L6928D
HIGH EFFICIENCY MONOLITHIC SYNCHRONOUS STEP DOWN REGULATOR
1

FEATURES
2V TO 5.5V BATTERY INPUT RANGE HIGH EFFICIENCY: UP TO 95% INTERNAL SYNCHRONOUS SWITCH NO EXTERNAL SCHOTTKY REQUIRED EXTREMELY LOW QUIESCENT CURRENT 1A MAX SHUTDOWN SUPPLY CURRENT 800mA MAX OUTPUT CURRENT ADJUSTABLE OUTPUT VOLTAGE FROM 0.6V LOW DROP-OUT OPERATION: UP TO100% 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
Figure 1. Packages
MSOP8
Table 1. Order Codes
Part Number L6928D L6928D013TR Package MSOP8 in Tube MSOP8 in Tape & Reel
3
DESCRIPTION
2

APPLICATIONS
BATTERY-POWERED EQUIPMENTS PORTABLE INSTRUMENTS CELLULAR PHONES PDAs AND HAND HELD TERMINALS DSC GPS
The device is dc-dc monolithic regulator specifically designed to provide extremely high efficiency. L6928D 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. L6928D is externally synchronizable with a clock which makes it useful in noisesensitive applications. Other features like Powergood, Overvoltage protection, Shortcircuit protection and Thermal Shutdown (150C) are also present.
Figure 2. Application Test Circuit
L 4.7H VIN=2V to 5.5V C1 10F 6.3V SYNC VCC RUN 5 7 6 1 2 COMP
D01IN1528
VOUT=1.8V R2 200K C4 10F 6.3V
LX
R3 500K
8 PGOOD 4 C2 220pF 3 VFB GND R1 100K
February 2005
Rev. 2 1/9
L6928D
Table 2. Absolute Maximum Ratings
Symbol V6 V5 V1 V3 V2 V8 V7 Ptot Tj Tstg LX Pin Other pins Input voltage Output switching voltage Shutdown Feedback voltage Error amplifier output voltage PGOOD Synchronization mode selector Power dissipation at Tamb=70C Junction operating temperature range Storage temperature range Maximum Withstanding Voltage Range Test Condition: CDFAEC-Q100-002- "Human Body Model" Acceptance Criteria: "Normal Performance' Parameter Value -0.3 to 6 -1 to VCC -0.3 to VCC -0.3 to VCC -0.3 to VCC -0.3 to VCC -0.3 to VCC 0.45 -40 to 150 -65 to 150 1000 2000 Unit V V V V V V V W C C V V
Figure 3. Pin Connection
RUN COMP VFB GND 1 2 3 4 8 7 6 5 PGOOD SYNC VCC LX
D01IN1239AMOD
Table 3. Thermal Data
Symbol Rth j-amb Parameter Thermal Resistance Junction to Ambient Value 180 Unit C/W
Table 4. Pin Functions
N 1 2 3 4 5 6 7 Name RUN COMP VFB GND LX VCC SYNC Description Shutdown input. When connected to a low level (lower than 0.4V) the device stops working. When high (higher than 1.3V) the device is enabled. Error amplifier output. A compensation network has to be connected to this pin. Usually a 220pF capacitor is enough to guarantee the loop stability. Error amplifier inverting input. The output voltage can be adjusted from 0.6V up to the input voltage by connecting this pin to an external resistor divider. Ground. Switch output node. This pin is internally connected to the drain of the internal switches. Input voltage. The start up input voltage is 2.2V (typ) while the operating input voltage range is from 2V to 5.5V. An internal UVLO circuit realizes a 100mV (typ.) hysteresis. Operating mode selector input. When high (higher than 1.3V) the Low Consumption Mode is selected. When low (lower than 0.5V) the Low Noise Mode is selected. If connected with an appropriate external synchronization signal (from 1MHz up to 2MHz) the internal synchronization circuit is activated and the device works at the same switching frequency. Power good comparator output. It is an open drain output. A pull-up resistor should be connected between PGOOD and VOUT (or VCC depending on the requirements). The pin is forced low when the output voltage is lower than 90% of the regulated output voltage and goes high when the output voltage is greater than 90% of the regulated output voltage. If not used the pin can be left floating.
8
PGOOD
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Table 5. Electrical Characteristics (Tj = 25C, VCC = 3.6V unless otherwise specified)
Symbol Vcc Vcc ON Vcc OFF Vcc hys Rp Rn Ilim Parameter Operating input voltage Turn On threshold Turn Off threshold Hysteresis High side Ron Low side Ron Peak current limit Valley current limit Vout fosc fsync Iq Output voltage range Oscillator frequency Sync mode clock (*) 1 Vcc = 3.6V, Ilx =100mA Vcc = 3.6V, Ilx =100mA Vcc = 3.6V Vcc = 3.6V Vfb 1.4 2 100 240 215 1.2 1.4 Vcc Test Condition After Turn on Min 2 2.2 2 Typ Max 5.5 Unit V V V mV m m A A V MHz MHz A A A A A
DC CHARACTERISTICS Quiescent current (low noise mode) Quiescent current (low cunsumption mode) Ish Ilx Shutdown current LX leakage current (*) Vsync = 0V, no load, VFB > 0.6V Vsync = Vcc, no load, VFB > 0.6V RUN to GND, Vcc = 5.5V RUN to GND, VLX = 5.5V, Vcc = 5.5V RUN to GND, VLX = 0V, Vcc = 5.5V ERROR AMPLIFIER CHARACTERISTICS Vfb Ifb RUN Vrun_H Vrun_L Irun Vsync_H Vsync_L VPGOOD VPGOOD VPgood(low) ILK-PGOOD RUN threshold high RUN threshold low RUN input current (*) 0.4 25 1.3 V V nA Voltage feedback Feedback input current (*) VFB = 0.6V 0.593 0.6 25 0.607 V nA 230 25 0.2 1 1
SYNC/MODE FUNCTION Sync mode threshold high Sync mode threshold low 0.5 1.3 V V
PGOOD SECTION Power Good Threshold Power Good Hysteresis Power Good Low Voltage Power Good Leakage Current (*) VOUT = Vfb VOUT = Vfb Run to GND VPGOOD = 3.6V 50 90 4 0.4 %Vout %Vout V nA
PROTECTIONS HOVP Hard overvoltage threshold VOUT = Vfb 10 %Vout
(*) Guaranteed by design
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4
OPERATION DESCRIPTION
The main loop uses slope compensated PWM current mode architecture. Each cycle the high side MOSFET is turned on, triggered by the oscillator, so that the current flowing through it (the same as the inductor current) increases. When this current reaches the threshold (set by the output of the error amplifier E/A), the peak current limit comparator PEAK_CL turns off the high side MOSFET and turns on the low side one until the next clock cycle begins or the current flowing through it goes down to zero (ZERO CROSSING comparator). The peak inductor current required to trigger PEAK_CL depends on the slope compensation signal and on the output of the error amplifier. In particular, the error amplifier output depends on the VFB pin voltage. When the output current increases, the output capacitor is discharged and so the VFB pin decreases. This produces increase of the error amplifier output, so allowing a higher value for the peak inductor current. For the same reason, when due to a load transient the output current decreases, the error amplifier output goes low, so reducing the peak inductor current to meet the new load requirements. The slope compensation signal allows the loop stability also in high duty cycle conditions (see related section) Figure 4. Device Block Diagram
SYNC RUN VCC
OSCILLATOR LOW NOISE/ CONSUM PTION LOOP CONTROL
GND
SENSE P MOS
POWER PMOS
COM P
SLOPE GND
FB
E/A
VREF
0.6V
PEAK CL
DRIVER OVP
LX
P GOOD VREF
0.9V
ZERO CROSSING GND
Vcc
SENSE NMOS
Vcc
POWER NMOS
P GOOD
VALLEY CL
GND
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 25A.
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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). 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:
( V IN - V OU T ) I O N = ---------------------------------- T O N L ( V OUT ) I O FF = ------------------ T O FF L
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:
V IN I PEAK = I VALL EY + -------- T ON_MIN L
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 In current mode architectures, when the duty cycle of the application is higher than approximately 50%, a pulseby-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.
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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.
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:
R2 V OUT = 0.6 1 + ------ R
1
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 150C. In this case both the high side MOSFET and the low side one are turned off. Once the junction temperature goes back lower than 95C, the device restarts the normal operation.
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Figure 5. MSOP8 Mechanical Data & Package Dimensions
mm DIM. MIN. A A1 A2 b c D (1) E 0.050 0.750 0.250 0.130 2.900 4.650 3.000 4.900 3.000 0.650 0.400 0.550 0.950 0 (min.) 6 (max.) 0.100 0.004 0.700 0.016 0.850 TYP. MAX. 1.10 0.150 0.950 0.400 0.230 3.100 5.150 3.100 0.002 0.03 0.010 0.005 0.114 0.183 0.114 0.118 0.193 0.118 0.026 0.022 0.037 0.028 0.033 MIN. TYP. MAX. 0.043 0.006 0.037 0.016 0.009 0.122 0.20 0.122 inch
OUTLINE AND MECHANICAL DATA
E1 (1) 2.900 e L L1 k aaa
Note: 1. D and F does not include mold flash or protrusions. Mold flash or potrusions shall not exceed 0.15mm (.006inch) per side.
MSOP8 (Body 3mm)
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Table 6. Revision History
Date October 2004 February 2005 Revision 1 2 First Issue. Changed from Product Preview to Final datasheet. Description of Changes
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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 (c) 2005 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 www.st.com
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