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LM2788MMX-2.0 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM2788MMX-2.0 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 12 page  Operation Description (Continued) holds true for the other base-gain regions. At the base-gain transitions (V IN = 2.9V, 3.8V), the 10mA curve makes sharps transition as the part switches base-gains. The 10mA load curve gives a clear picture of how base-gain affects overall converter efficiency. With a 10mA output current, the gain of the LM2788-1.8 is equal to the base-gain over the entire operating input voltage range. Additionally, with a 10mA load, internal supply current has a minimal impact on efficiency (Supply current does have a small affect: it is why the 10mA load curve is slightly below the ideal efficiency gradients in each of the 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, how- ever. This output droop is typically modeled as an output resistance, R OUT, because the magnitude of the droop in- creases linearly with load current. Ideal Charge Pump: V OUT =GxVIN Real Charge Pump: V OUT =(GxVIN)-(IOUT xROUT) The LM2788 compensates for output voltage droop under high load conditions by gain hopping: when the base-gain is not sufficient to keep the output voltage in regulation, the part will temporarily switch 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. 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 efficiency curve of the LM2788-1.8 with a 120mA output current, also contained in Figure 1, shows the effect that gain hopping has on efficiency. Comparing the 120mA load curve to the 10mA load curve, it is plain to see that to the right of the base-gain transitions, the efficiency of the 120mA curve increases gradually whereas the 10mA curve makes a 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 will spend a percentage of time in the base-gain setting and the rest of the time in the next-highest gain setting. The percentage of time gain hopping decreases as the input voltage rises, as less gain-hopping boost is required with increased input voltage. When the input volt- age in a given base-gain region is large enough so that no extra boost from gain hopping is required, the 120mA-load efficiency curve mirrors the 10mA efficiency curve. TABLE 2. LM2788-1.8 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 LM2788. Gain hopping only occurs when required for keep- ing the output voltage in regulation. This allows the LM2788 to operate in the higher efficiency base-gain setting as much as possible. Gain hopping also allows the base-gain transi- tions to be placed at input voltages that are as low as practically possible. This maximizes the peaks, and mini- mizes the valleys, of the efficiency ’saw-tooth’ curves, again maximizing total solution efficiency. SHUTDOWN The LM2788 is in shutdown mode when the voltage on the active-low logic enable pin (EN) is low. In shutdown, the LM2788 draws virtually no supply current. When in shut- down, the output of the LM2788 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 LM2788 and drive the output pin with another voltage source. This can be done, but the voltage on the output pin of the LM2788 must not be brought above the input voltage. The output pin will draw a small amount when driven exter- nally due the internal feedback resistor divider connected between V OUT and GND. SOFT START The LM2788 employs soft start circuitry to prevent excessive input inrush currents during startup. The output voltage is programmed to rise from 0V to the nominal output voltage in approximately 400µs (typ.). With the input voltage estab- lished, soft-start is engaged when a part is enabled by pulling the voltage on the EN pin high. Soft-start also en- gages when voltage is established simultaneously to the V IN and EN pins THERMAL SHUTDOWN Protection from overheating-related damage is achieved with a thermal shutdown feature. When the junction tem- perature rises to 150oC (typ.), the part switches into shut- down mode. The LM2788 disengages thermal shutdown when the junction temperature of the part is reduced to 130oC (typ.). Due to its high efficiency, the LM2788 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 LM2788 short-circuit protection circuitry that 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 LM2788 is highly dependent on the application conditions. The output capaci- tor, the input voltage, and the output current each play a significant part in determining the output voltage ripple. Due to the complexity of LM2788 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 www.national.com 9 |
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