LM2798

LM2797/LM2798 120mA High Efficiency Step-Down Switched Capacitor Voltage Converter with Voltage Monitoring April 2003 LM2797/LM2798 120mA High Efficiency Step-Down Switched Capacitor Voltage Converter with Voltage Monitoring General Description The LM2797/98 switched capacitor step-down DC/DC converters efficiently produce a 120mA regulated low-voltage rail from a 2.6V to 5.5V input. Fixed output voltage options of 1.5V, 1.8V, and 2.0V are available. The LM2797/98 uses multiple fractional gain configurations to maximize conversion efficiency over the entire input voltage and output current ranges. Also contributing to high overall efficiency is the extremely low supply current of the LM2797/98: 35µA operating unloaded and 0.1µA in shutdown. Features of the LM2797/98 include input voltage and output voltage monitoring. Pin BATOK provides battery monitoring by indicating when the input voltage is above 2.85V (typ.). Pin POK verifies that the output voltage is not more than 5% (typ.) below the nominal output voltage of the part. The optimal external component requirements of the LM2797/98 solution minimize size and cost, making the part ideal for Li-Ion and other battery powered designs. Two 1µF flying capacitors and two 10µF bypass capacitors are all that is required, and no inductors are needed. The LM2797/98 also features short-circuit protection overtemperature protection, and soft-start circuitry to prevent excessive inrush currents. The LM2798 has a 400µs turn-on time. The turn-on time of the LM2797 is 100µs. Features n Output voltage options: 2.0V ± 5%, 1.8V ± 5%, and 1.5V ± 6% n 120mA output current capability n Multi-Gain and Gain Hopping for Highest Possible Efficiency - up to 90% Efficient n 2.6V to 5.5V input range n Input and Output Voltage Monitoring (BATOK and POK) n Low operating supply current: 35µA n Shutdown supply current: 0.1µA n Thermal and short circuit protection n LM2798 turn-on time: 400µs LM2797 turn-on time: 100µs n Available in an 10-Pin MSOP Package Applications n n n n n Cellular Phones Pagers H/PC and P/PC Devices Portable Electronic Equipment Handheld Instrumentation Typical Application Circuit 20044501 © 2003 National Semiconductor Corporation DS200445 www.national.com LM2797/LM2798 Connection Diagram LM2797/98 Mini SO-10 (MSOP-10) Package NS Package #: MUB10A 20044502 Top View Pin Description Pin 1 2 3 4 5 Name VOUT C1C1+ VIN POK Regulated Output Voltage First Flying Capacitor: Negative Terminal First Flying Capicitor: Positive terminal Input Voltage. Recommended VIN Range: 2.6V to 5.5V Power-OK Indicator: Output voltage sense. Open-drain NFET output. With an external pull-up resistor tied to POK, V(POK) will be high when VOUT is regulating correctly. When VOUT falls out of regulation, the internal open-drain FET pulls the POK voltage low. Battery-OK Indicator: Input voltage sense. Open-drain NFET output. With an external pull-up resistor tied to BATOK, V(BATOK) will be high when VIN > 2.85V (typ). LM2797/98 pulls V(BATOK) low when VIN < 2.65V (typ.) , and/or when the part is in shutdown [V(EN) = 0]. Enable Logic Input. High voltage = ON, Low voltage = SHUTDOWN Second Flying Capacitor: Positive Terminal Ground Connection Second Flying Capacitor: Negative Terminal Description 6 BATOK 7 8 9 10 EN C2+ GND C2- Ordering Information Nominal Turn-on Output Time Voltage VOUT(NOM) 1.80V 1.50V 1.80V 2.00V 100µs 400µs 400µs 400µs Order Number Package Marking Supplied As: LM2797MM-1.8 LM2797MMX-1.8 LM2798MM-1.5 LM2798MMX-1.5 LM2798MM-1.8 LM2798MMX-1.8 LM2798MM-2.0 LM2798MMX-2.0 S80B S56B S57B S58B 1000 units on Tape-and Reel 3500 units on Tape-and-Reel 1000 units on Tape-and Reel 3500 units on Tape-and-Reel 1000 units on Tape-and Reel 3500 units on Tape-and-Reel 1000 units on Tape-and Reel 3500 units on Tape-and-Reel www.national.com 2 LM2797/LM2798 Absolute Maximum Ratings 2) (Notes 1, Operating Ratings Input Voltage Range Recommended Output Current Range Junction Temperature Range Ambient Temperature Range (Note 6) (Notes 1, 2) 2.6V to 5.5V 0mA to 120mA -40˚C to 125˚C -40˚C to 85˚C If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN, EN, POK, BATOK pins: Voltage to Ground (Note 3) Junction Temperature (TJ-MAX-ABS) Continuous Power Dissipation (Note 4) VOUT Short-Circuit to GND Duration (Note 4) Storage Temperature Range Lead Temperature (Soldering, 5 Sec.) ESD Rating (Note 5) Human-body model: Machine model −0.3V to 5.6V 150˚C Internally Limited Unlimited −65˚C to 150˚C 260˚C 2 kV 200V (Notes 2, 8) Thermal Information Thermal Resistance, MSOP-8 Resistance, MSOP-8 Package (θJA) (Note 7) 220˚C/W Electrical Characteristics Symbol Parameter Limits in standard typeface and typical values apply for TJ = 25oC. Limits in boldface type apply over the operating junction temperature range. Unless otherwise specified: 2.6 ≤ VIN ≤ 5.5V, V(EN) = VIN, C1 = C2 = 1µF, CIN = COUT = 10µF. (Note 9) Conditions 2.8V ≤ VIN ≤ 4.2V 0mA ≤ IOUT ≤ 120mA 4.2V < VIN ≤ 5.5V 0mA ≤ IOUT ≤ 120mA 2.8V ≤ VIN ≤ 4.2V 0mA ≤ IOUT ≤ 120mA 4.2V < VIN ≤ 5.5V 0mA ≤ IOUT ≤ 120mA IOUT = 0mA V(EN) = 0V LM2798-1.8: VIN = 3.6V, IOUT = 120mA LM2798-1.8: VIN = 3.0V, IOUT = 60mA LM2798-1.5: 3.0 ≤ VIN ≤ 4.2V, IOUT = 60mA LM2798-1.8: 3.0 ≤ VIN ≤ 4.2V, IOUT = 60mA LM2798-2.0: 3.0 ≤ VIN ≤ 4.2V, IOUT = 60mA LM2798, VIN=2.6V, IOUT=100mA, (Note 12) LM2797, VIN=2.6V, IOUT=100mA, (Note 12) fSW ISC VIH VIL IEN Switching Frequency Short-Circuit Current EN pin Logic-High Input EN pin Logic-Low Input EN pin input current VEN = 0V VEN = 5.5V VIN = 3.6, VOUT = 0V 0.9 0 0 30 Min -5 -6 Typ Max +5 +6 Units LM2797-1.8, LM2798-1.8, LM2798-2.0 % of VOUT(nom) (Note 10) VOUT Output Voltage Tolerance LM2798-1.5 -6 -6 +6 +6 % of VOUT(nom) (Note 10) VOUT Output Voltage Tolerance All Output Voltage Options IQ I SD Operating Supply Current Shutdown Supply Current Output Voltage Ripple Peak Efficiency Average Efficiency over Li-Ion Input Voltage Range (Note 11) Turn-On Time 35 0.1 20 90 76 82 75 400 100 500 25 50 2 µA µA mVp-p % % VR EPEAK EAVG tON µs kHz mA VIN 0.4 V V nA Enable Pin (EN) Characteristics 3 www.national.com LM2797/LM2798 Electrical Characteristics Symbol VT-POK Parameter Threshold of output voltage for POK transition POK-high leakage current POL-low pull-down voltage Input voltage threshold for BATOK transition BATOK-high leakage current BATOK-low pull-down voltage (Notes 2, 8) (Continued) Limits in standard typeface and typical values apply for TJ = 25oC. Limits in boldface type apply over the operating junction temperature range. Unless otherwise specified: 2.6 ≤ VIN ≤ 5.5V, V(EN) = VIN, C1 = C2 = 1µF, CIN = COUT = 10µF. (Note 9) Conditions POK transition L to H POK transition H to L Hysterisis IPOK-H VPOK-L VT-BATOK V(POK) = 3.6V I(POK) = -100µA BATOK transition L to H BATOK transition H to L Hysterisis IBATOK-H VBATOK-L V(BATOK) = 3.6V I(BATOK) = - 100µA 2.4 83 Min Typ 95 92 3 1 200 2.85 2.65 0.20 1 200 5 300 µA mV 5 300 3.0 Max 99 Units % of VOUT-NOM (Note 10) µA mV V POK Characteristics BATOK Characteristics Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics tables. Note 2: All voltages are with respect to the potential at the GND pin. Note 3: Voltage on the EN pin must not be brought above VIN + 0.3V. Note 4: Thermal shutdown circuitry protects the device from permanent damage. Note 5: The human-body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. Note 6: Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 125oC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP - (θJA x PD-MAX). The ambient temperature operating rating is provided merely for convenience. This part may be operated outside the listed TA rating so long as the junction temperature of the device does not exceed the maximum operating rating of 125oC. Note 7: Junction-to-ambient thermal resistance is highly dependent on application conditions and PC board layout. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues. For more information on these topics, please refer to the Power Dissipation section of this datasheet. Note 8: All room temperature limits are 100% tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed by correlation using standard Statistical Quality Control methods (SQC). All limits are used to calculate Average Outgoing Quality Level (AOQL). Typical numbers are not guaranteed, but do represent the most likely norm. Note 9: CIN, COUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics Note 10: VOUT (NOM) is the nominal output voltage of the part. An example: VOUT-NOM of LM2798MM-1.8 is 1.8V. Note 11: Efficiency is measured versus VIN, with VIN being swept in small increments from 3.0V to 4.2V. The average is calculated from these measurement results. Weighting to account for battery voltage discharge characteristics (VBAT vs. Time) is not done in computing the average. Note 12: Turn-on time is measured from when the EN signal is pulled high until the output voltage crosses 90% of its final value. Resistive load used for startup measurement, with value chosen to give IOUT = 100mA when the output voltage is fully established. www.national.com 4 LM2797/LM2798 Block Diagram 20044503 5 www.national.com LM2797/LM2798 Typical Performance Characteristics Unless otherwise specified: CIN = 10µF, C1 = 1.0µF, C2 = 1.0µF COUT = 10µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). Output Voltage vs. Input Voltage: LM2798-1.5 (1mA) Output Voltage vs. Input Voltage: LM2798-1.5 (120mA) 20044507 20044508 Output Voltage vs. Input Voltage: LM2797/98-1.8 (1mA) Output Voltage vs. Input Voltage: LM2797/98-1.8 (120mA) 20044509 20044510 Output Voltage vs. Input Voltage: LM2798-2.0 (1mA) Output Voltage vs. Input Voltage: LM2798-2.0 (120mA) 20044511 20044512 www.national.com 6 LM2797/LM2798 Typical Performance Characteristics Unless otherwise specified: CIN = 10µF, C1 = 1.0µF, C2 = 1.0µF COUT = 10µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued) Efficiency vs. Input Voltage: LM2798-1.5 Efficiency vs. Output Current: LM2798-1.5 20044513 20044514 Efficiency vs. Input Voltage: LM2797/98-1.8 Efficiency vs. Output Current: LM2797/98-1.8 20044515 20044516 Efficiency vs. Input Voltage: LM2798-2.0 Effiency vs. Output Current: LM2798-2.0 20044517 20044518 7 www.national.com LM2797/LM2798 Typical Performance Characteristics Unless otherwise specified: CIN = 10µF, C1 = 1.0µF, C2 = 1.0µF COUT = 10µF, TA = 25oC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued) Output Voltage Ripple vs. Output Current Output Voltage Ripple vs. Input Voltage 20044521 20044519 Output Voltage Ripple Short Circuit Current 20044506 20044520 Start Up Waveform: LM2798-1.8 Transient Load Response 20044504 20044505 www.national.com 8 LM2797/LM2798 Operation Description OVERVIEW The LM2797/98 are switched capacitor converters that produce a regulated low-voltage output. The core of the parts is a highly efficient charge pump that utilizes multiple fractional gains and pulse-frequency modulated (PFM) switching to minimize power losses over wide input voltage and output current ranges. A description of the principal operational characteristics of the LM2797/98 is broken up into the following sections: PFM Regulation, Fractional Multi-Gain Charge Pump, and Gain Selection for Optimal Efficiency. Each of these sections refers to the block diagram presented on the previous page. PFM REGULATION The LM2797/98 achieves tightly regulated output voltages with pulse-frequency modulated (PFM) regulation. PFM simply means the part only pumps when it needs to. When the output voltage is above the target regulation voltage, the part idles and consumes minimal supply-current. In this state, the load current is supplied solely by the charge stored on the output capacitor. As this capacitor discharges and the output voltage falls below the target regulation voltage, the charge pump activates. Charge/current is delivered to the output (supplying the load and boosting the voltage on the output capacitor). The primary benefit of PFM regulation is when output currents are light and the part is predominantly in the lowsupply-current idle state. Net supply current is minimal because the part only occasionally needs to recharge the output capacitor by activating the charge pump. FRACTIONAL MULTI-GAIN CHARGE PUMP The core of the LM2797/98 is a two-phase charge pump controlled by an internally generated non-overlapping clock. The charge pump operates by using the external flying capacitors, C1 and C2, to transfer charge from the input to the output. During the charge phase, which doubles as the PFM "idle state", the flying capacitors are charged by the input supply. The charge pump will be in this state until the output voltage drops below the target regulation voltage, triggering the charge pump to activate so that it can deliver charge to the output. Charge transfer is achieved in the pump phase. In this phase, the fully charged flying capacitors are connected to the output so that the charge they hold can supply the load current and recharge the output capacitor. Input, output, and intermediary connections of the flying capacitors are made with internal MOS switches. The LM2797/98 utilizes two flying capacitors and a versatile switch network to achieve several fractional voltage gains: 1⁄2, 2⁄3, and 1. With this gain-switching ability, it is as if the LM2797/98 is three-charge-pumps-in-one. The "active" charge pump at any given time is the one that will yield the highest efficiency given the input and output conditions present. GAIN SELECTION AND GAIN HOPPING FOR OPTIMAL EFFICIENCY The ability to switch gains based on input and output conditions results in optimal efficiency throughout the operating ranges of the LM2797/98. Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are neglected for simplicity): IIN = G x IOUT E = (VOUT x IOUT) ÷ (VIN x IIN) = VOUT ÷ (G X VIN) In the equations, G represents the charge pump gain. Efficiency is at its highest as GxVIN approaches VOUT. Optimal efficiency is achieved when gain is able to adjust depending on input and output voltage conditions. Due to the nature of charge pumps, G cannot adjust continuously, which would be ideal from an efficiency standpoint. But G can be a set of simple quantized ratios, allowing for a good degree of efficiency optimization. The gain set of the LM2797/98 consists of the gains 1/2, 2/3, and 1. An internal input voltage range detector, along with the nominal output voltage of a given LM2797/98 option, determines what is to be referred to as the "base gain" of the part, GB. The base gain is the default gain configuration of the part over a set VIN range. Table 1 lists GB of the LM27981.8 over the input voltage range. For the remainder of this discussion, the 1.8V option of the LM2798 will be used as an example. The other voltage options of the LM2798 operate under the same principles as LM2798-1.8, the gain transitions merely occur at different input voltages. Since the only difference between the LM2797 and the LM2798 is start-up time, the modes of operation of the LM2798-1.8 discussed here are identical to those of the LM2797-1.8. TABLE 1. LM2798-1.8 Base Gain (GB) vs. VIN Input Voltage 2.6V - 2.9V 2.9V - 3.8V 3.8V - 5.5V Base Gain (GB) 1 23 12 ⁄ ⁄ Figure 1 shows the efficiency of the LM2798-1.8 versus input voltage, with output currents of 10mA and 120mA. The base gain regions (GB) are separated and labeled. There is also a set of ideal efficiency gradients, EIDEAL(G=xx) , showing the ideal efficiency of a charge pumps with gains of 1/2, 2/3, and 1. These gradients have been generated using the ideal efficiency equation presented above. 20044522 FIGURE 1. Efficiency of LM2798-1.8 with 10mA and 120mA output currents. Base-gain (GB) regions are separated and labeled. Ideal efficiency curves of charge pumps with G =1/2, 2/3, and 1 are included, and are labelled: EIDEAL(G=1), EIDEAL(G=2/3), EIDEAL(G=1/2) 9 www.national.com LM2797/LM2798 Operation Description (Continued) The 10mA load curve in Figure 1 gives a clear picture of how base-gain affects overall converter efficiency. The "ideal efficiency gradients" in the figure show the efficiency of ideal switched capacitor converters with gains of 1, 2/3, and 1/2, respectively. The 10mA-load efficiency curve closely follows the ideal efficiency gradients in each of the respective basegain regions. At the base-gain transitions (VIN = 2.9V, 3.8V), there are sharp transitions in the 10mA curve because the LM2797/98 switches base-gains. With a 10mA output current there is very little gain hopping (described below), and the gain of the LM2798-1.8 is equal to the base-gain over the entire operating input voltage range. Internal supply current has a minimal impact on efficiency with a 10 mA load. Supply current does have a small effect, and it the reason why the 10mA load curve is slightly below the ideal efficiency gradients in each of the base-gain regions. But overall, due to the lack of gain hopping and the minimal impact of supply current on converter efficiency, the 10mA load curve very closely mirrors the ideal efficiency curves in each of the respecitve base-gain regions. The 120mA-load curve in Figure 1 illustrates the effect of gain hopping on converter efficiency. Gain hopping is implemented to overcome output voltage droop that results from charge-pump non-idealities. In an ideal charge pump, the output voltage is equal to the product of the gain and the input voltage. Non-idealities such as finite switch resistance, capacitor ESR, and other factors result in the output of practical charge pumps being below the ideal value. This output droop is typically modeled as an output resistance, ROUT, because the magnitude of the droop increases linearly with load current. Ideal Charge Pump: VOUT = G x VIN Real Charge Pump: VOUT = (G x VIN) - (IOUT x ROUT) The LM2797/98 compensates for output voltage droop under high load conditions by gain hopping. When the basegain is not sufficient to keep the output voltage in regulation, the part will temporarily hop up to the next highest gain setting to provide an intermittent boost in output voltage. When the output voltage is sufficiently boosted, the gain configuration reverts back to the base-gain setting. An example: if the input voltage of the LM2798-1.8 is 3.2V, the part is in the 2/3 base-gain region. If the output voltage droops, the gain configuration will temporarily hop up to a gain of 1. It will operate with a gain of 1 until the nominal output voltage is restored, at which time the gain will hop back down to 2/3. If the load remains high, the part will continue to hop back and forth between the base-gain and the next highest gain setting, and the output voltage will remain in regulation. In contrast to the base-gain decision, which is made based on the input voltage, the decision to gain hop is made by monitoring the voltage at the output of the part. The 120mA-load efficiency curve in Figure 1 illustrates the effect of gain hopping on efficiency. Comparing the 120mA load curve to the 10mA load curve, notice that to the right of the base-gain transitions the efficiency of the 120mA curve increases gradually. In contrast, the 10mA curve makes a very sharp transition. The base-gain of both curves is the same for both loads. The difference comes in gain hopping. With the 120mA load, the part operates in the base-gain setting for a certain percentage of time and in the nexthighest gain setting for the remainder. The percentage of time spent in an elevated gain configuration decreases as the input voltage rises, as less gain-hopping boost is required with increased input voltage. When the input voltage in a given base-gain region is large enough so that no extra boost from gain hopping is required, the part operates entirely in the base gain region. This can be seen in the figure where the 120mA-load efficiency curve follows the ideal efficiency gradients. TABLE 2. LM2798-1.8 Gain Hopping Regions Input Voltage 3.0V - 3.3V 3.8V - 4.4V Base Gain (GB) 23 12 Gain Hop Setting 1 23 ⁄ ⁄ ⁄ Gain hopping contributes to the overall high efficiency of the LM2797/98. Gain hopping only occurs when required to keep the output voltage in regulation. This allows the LM2797/98 to operate in the higher efficiency base-gain setting as much as possible. Gain hopping also allows the base-gain transitions to be placed at input voltages that are as low as practically possible. Doing so maximizes the peaks and minimizes the valleys of the efficiency "saw-tooth" curves, maximizing total solution efficiency. POK: OUTPUT VOLTAGE STATUS INDICATOR The POK pin is an NMOS-open-drain-logic signal that indicates when the output voltage of the LM2797/98 is at or above 95% (typ) of the target output voltage. To function properly, the POK pin must be connected to a pull-up resistor (1MΩ (typ.)), or other pull-up device. With a pull-up in place, V(POK) will be HIGH when VOUT is at or above 95% (typ) of the nominal output voltage (VOUT-nom = 1.5V, 1.8V, or 2.0V, depending on voltage option). If the output falls below 92% (typ.) of the nominal output voltage, V(POK) will be 0V. There is hysteresis of 3% between the thresholds. The POK function is disabled and V(POK) is pulled down to 0V when the LM2797/98 is in shutdown (EN = 0V). Table 3 is a complete list of the typical POK regions of operation. TABLE 3. Typical POK functionality, with 1MΩ pull-up resistor connected between POK and VOUT VIN EN H H L X VOUT POK State HIGH LOW LOW LOW Internal POK Transistor State OFF ON ON OFF V(POK) VOUT 0V 0V 0V, (VOUT off) > 1.7V > 1.7V > 1.7V < 1.7V > 95% of VOUT-nom ≤ 92% OF VOUT-nom X X www.national.com 10 LM2797/LM2798 Operation Description VIN (Continued) TABLE 4. Typical BATOK functionality, with 1MΩ pull-up resistor connected between BATOK and VIN EN H H L X BATOK State HIGH LOW LOW LOW Internal BATOK Transistor State OFF ON ON OFF V(BATOK) VIN 0V 0V VIN, ≤ 1.1V > 2.85V > 1.1V, < 2.65V > 1.1V ≤ 1.1V BATOK: INPUT VOLTAGE STATUS INDICATOR The BATOK pin is an NMOS-open-drain-logic signal that indicates the status of the input voltage. To function properly, the BATOK pin must be connected to a pull-up resistor, or other pull-up device. With a pull-up in place, V(BATOK) will be HIGH when VIN is at or above 2.85V. If the output falls below 2.65V (typ.), V(BATOK) will be 0V. There is hysteresis of 20mV (typ.) between the thresholds. The BATOK function is disabled and V(BATOK) is pulled down to 0V when the LM2797/98 is in shutdown (EN = 0V). Table 4 is a complete list of the typical BATOK regions of operation. SHUTDOWN The LM2797/98 is in shutdown mode when the voltage on the active-low logic enable pin (EN) is low. In shutdown, the LM2797/98 draws virtually no supply current. When in shutdown, the output of the LM2797/98 is completely disconnected from the input, and will be 0V unless driven by an outside source. In some applications, it may be desired to disable the LM2797/98 and drive the output pin with another voltage source. This can be done, but the voltage on the output pin of the LM2797/98 must not be brought above the input voltage. The output pin will draw a small amount of current when driven externally due the internal feedback resistor divider connected between VOUT and GND. SOFT START The LM2797/98 employs soft start circuitry to prevent excessive input inrush currents during startup. At startup, the output voltage gradually rises from 0V to the nominal output voltage. This occurs in 400µs (typ.) with the LM2798. Turn-on time of the LM2797 is 100µs (typ.). Soft-start is engaged when the part is enabled, including situations where voltage is established simultaneously on the VIN and EN pins. THERMAL SHUTDOWN Protection from overheating-related damage is achieved with a thermal shutdown feature. When the junction temperature rises to 150oC (typ.), the part switches into shutdown mode. The LM2797/98 disengages thermal shutdown when the junction temperature of the part is reduced to 130oC (typ.). Due to its high efficiency, the LM2797/98 should not activate thermal shutdown (or exhibit related thermal cycling) when the part is operated within specified input voltage, output current, and ambient temperature operating ratings. SHORT-CIRCUIT PROTECTION The LM2797/98 short-circuit protection circuitry protects the device in the event of excessive output current and/or output shorts to ground. A graph of "Short-Circuit Current vs. Input Voltage" is provided in the Performance Characteristics section. Application Information OUTPUT VOLTAGE RIPPLE The voltage ripple on the output of the LM2797/98 is highly dependent on application conditions. The output capacitor, the input voltage, and the output current each play a significant part in determining the output voltage ripple. Due to the complexity of LM2797/98 operation, providing equations or models to approximate the magnitude of the ripple cannot be easily accomplished. The following general statements can be made, however The output capacitor will have a significant effect on output voltage ripple magnitude. Ripple magnitude will typically be linearly proportional to the output capacitance present. A low-ESR ceramic capacitor is recommended on the output to keep output voltage ripple low. Placing multiple capacitors in parallel can reduce ripple significantly. Doing this increases capacitance and reduces ESR (the effective net ESR is governed by the properties of parallel resistance). Placing two identical capacitors in parallel have twice the capacitance and half the ESR, as compared to one of these capacitors all by itself. Similarly, if a large-value, high-ESR capacitor (tantalum, for example) is to be used as the primary output capacitor, the net output ESR can be significantly reduced by placing a low-ESR ceramic capacitor in parallel with this primary output capacitor. Ripple is increased when the LM2797/98 is gain hopping. With high output currents, ripple is likely to vary significantly with input voltage, depending on whether on not the part is gain hopping. CAPACITORS The LM2797/98 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2797/98 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use with the LM2797/98. These capacitors have tight capacitance tolerance (as good as ± 10%) and hold their value over temperature (X7R: ± 15% over -55oC to 125oC; X5R: ± 15% over -55oC to 85oC). 11 www.national.com LM2797/LM2798 Application Information (Continued) Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2797/98. These types of capacitors typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30oC to +85oC range; Z5U: +22%, -56% over +10oC to +85oC range). Under some conditions, a 1µF-rated Y5V or Z5U capacitor could have a capacitance as low as 0.1µF. Such detrimental deviation is likely to cause these Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2797/98. The table below lists some leading ceramic capacitor manufacturers. Manufacturer AVX Murata Taiyo-Yuden TDK Vishay-Vitramon Contact Information www.avx.com www.murata.com www.t-yuden.com www.component.tdk.com www.vishay.com Low-ESR ceramic capacitors with X7R or X5R temperature characteristic are strongly recommended for use here. The flying capacitors C1 and C2 should be identical. As a general rule, the capacitance value of each flying capacitor should be 1/10th that of the output capacitor. ESR should be as low as possible to minimize the output resistance of the charge pump and give maximum output current capability. Polarized capacitor (tantalum, aluminum electrolytic, etc.) must not be used for the flying capacitors, as they could become reversebiased upon start-up of the LM2797/98. INPUT CAPACITOR The input capacitor (CIN) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying capacitors during the charge phase of operation. The input capacitor helps to keep the input voltage from drooping at the start of the charge phase when the flying capacitor is connected to the input, and helps to filter noise on the input pin that could adversely affect sensitive internal analog circuitry biased off the input line. An X7R/X5R ceramic capacitor is recommended for use. As a general recommendation, the input capacitor should be chosen to match the output capacitor. POWER DISSIPATION LM2797/98 power dissipation will, typically, not be much of a concern in most applications. Derating to accommodate selfheating will rarely be required due to the high efficiency of the part. Peak power dissipation (PD) of all LM2797/98 options is seen with the LM2798-1.5 operating at VIN = 5.5V and IOUT = 120mA (conditions limited to valid operating ratings). Under these conditions, the power efficiency (E) of the LM2798-1.5 is 54% (typ.). Assuming a typical junctionto-ambient thermal resistance (θJA) for the MSOP package of 220˚C/Watt, the junction temperature (TJ) of the part is calculated below for a part operating at the maximum rated ambient temperature (TA) of 85˚C. PD = PIN - POUT = (POUT/E) - POUT = [(1/E) - 1] x POUT = [(1/64%) - 1] x 1.5V x 120mW = 153mW TJ = TA = (PD x θJA) = 85˚C + (.153W x 220˚C/W) =119˚C Even under these peak power dissipation and ambient temperature conditions, the junction temperature of the LM27981.5 is below the maximum operating rating of 125˚C. As an additional note, the ambient temperature operating rating range listed in the specifications is provided merely for convenience. The LM2797/98 may be operated outside this rating, so long as the junction temperature of the device does not exceed the maximum operating rating of 125˚C. OUTPUT CAPACITOR The output capacitor of the LM2797/98 greatly affect performance of the circuit. In typical high-current applications, a 10µF low-ESR (ESR = equivalent series resistance) ceramic capacitor is recommended. For lighter loads, the output capacitance may be reduced (as low as 1µF for output currents ≤ 60mA is usually acceptable). The performance of the part should be evaluated with special attention paid to efficiency and output ripple to ensure the capacitance chosen on the output yields performance suitable for the application. In extreme cases, excessive ripple could cause control loop instability, severely affecting the performance of the part. If excessive ripple is present, the output capacitance should be increased. The ESR of the output capacitor affects charge pump output resistance, which plays a role in determining output current capability. Both output capacitance and ESR affect output voltage ripple (See Output Voltage Ripple section, above). For these reasons, a low-ESR X7R/X5R ceramic capacitor is the capacitor of choice for the LM2797/98 output. FLYING CAPACITORS The flying capacitors (C1 and C2) transfer charge from the input to the output, and determine the strength of the charge pump: the larger the capacitance, the greater the output current capability. If capacitors are too small, the LM2797/98 could spend excessive amount of time gain hopping: decreasing efficiency, increasing output voltage ripple, and possibly impeding the ability of the part to regulate. On the other hand, if the flying capacitors are too large they could potentially overwhelm the output capacitor, resulting in increased output voltage ripple. www.national.com 12 LM2797/LM2798 Layout Guidelines Proper board layout to accommodate the LM2797/98 circuit will help to ensure optimal performance. The following guidelines are recommended: • Place capacitors as close to the LM2797/98 as possible, and preferably on the same side of the board as the IC. • Use short, wide traces to connect the external capacitors to the LM2797/98 to minimize trace resistance and inductance. • Use a low resistance connection between ground and the GND pin of the LM2797/98. Using wide traces and/or multiple vias to connect GND to a ground plane on the board is most advantageous. Figure 2 is a sample single-layer board layout that accommodates the LM2797/98 typical application circuit, as pictured on the cover of this datasheet 20044524 FIGURE 2. Sample single-layer board layout of the LM2797/98 Typical Application Circuit (Vias to a ground plane, assumed to be present, are located in the center of the LM2797/98 footprint.) 13 www.national.com LM2797/LM2798 120mA High Efficiency Step-Down Switched Capacitor Voltage Converter with Voltage Monitoring Physical Dimensions inches (millimeters) unless otherwise noted Mini SOP-10 (MSOP-10) MUB10A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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