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LM2798MMX-1.8 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2798MMX-1.8 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Operation Description (Continued) The 10mA load curve in Figure 1 gives a clear picture of how base-gain affects overall converter efficiency. The "ideal ef- ficiency 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 base- gain regions. At the base-gain transitions (V IN = 2.9V, 3.8V), there are sharp transitions in the 10mA curve because the LM2797/98 switches base-gains. With a 10mA output cur- rent 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 gradi- ents in each of the base-gain regions. But overall, due to the lack of gain hopping and the minimal impact of supply cur- rent 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 imple- mented 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, R OUT, because the magnitude of the droop increases lin- early with load current. Ideal Charge Pump: V OUT =GxVIN Real Charge Pump: V OUT =(GxVIN)-(IOUT xROUT) The LM2797/98 compensates for output voltage droop un- der high load conditions by gain hopping. When the base- gain 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 ex- ample: 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 next- highest 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 re- quired 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 en- tirely 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 Base Gain (G B) Gain Hop Setting 3.0V - 3.3V 2 ⁄3 1 3.8V - 4.4V 1 ⁄2 2 ⁄3 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 indi- cates 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 (1M Ω (typ.)), or other pull-up device. With a pull-up in place, V(POK) will be HIGH when V OUT is at or above 95% (typ) of the nominal output voltage (V OUT-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 func- tion 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 1M Ω pull-up resistor connected between POK and V OUT V IN EN V OUT POK State Internal POK Transistor State V(POK) >1.7V H >95% of V OUT-nom HIGH OFF V OUT >1.7V H ≤ 92% OF V OUT-nom LOW ON 0V >1.7V L X LOW ON 0V <1.7V X X LOW OFF 0V, (V OUT off) www.national.com 10 |
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