Datasheet LT1977 (Analog Devices) - 9

LT1977
W BLOCK DIAGRA
power from the VIN pin, but if the BIAS pin is connected to To further optimize efficiency, the LT1977 automatically an external voltage higher than 3V bias power will be switches to Burst Mode operation in light load situations. drawn from the external source (typically the regulated In Burst Mode operation, all circuitry associated with output voltage). This improves efficiency. controlling the output switch is shut down reducing the input supply current to 45µA. High switch efficiency is achieved by using the BOOST pin to provide a voltage to the switch driver which is higher The LT1977 contains a power good flag with a program- than the input voltage, allowing switch to be saturated. mable threshold and delay time. A logic-level low on the This boosted voltage is generated with an external capaci- SHDN pin disables the IC and reduces input suppy current tor and diode. to less than 1µA.
U U W U APPLICATIO S I FOR ATIO CHOOSING THE LT1976 OR LT1977
the LT1977’s input range will be similar to the LT1976. Lowering the input voltage below the maximum duty cycle The LT1976 and LT1977 are both high voltage 1.5A step- limitation will cause a dropout in regulation. down Burst Mode switching regulators with a typical quiescent current of 100µA. The difference between the
Table 1. LT1976/LT1977 Comparison
two is that the fixed switching frequency of the LT1976 is
PARAMETER ADVANTAGE
200kHz versus 500kHz for the LT1977. The switching Minimum Duty Cycle LT1976 frequency affects: inductor size, input voltage range in Maximum Duty Cycle LT1976 continuous mode operation, efficiency, thermal loss and Inductor Size LT1977 EMI. Output Capacitor Size LT1977 Efficiency LT1976
OUTPUT RIPPLE AND INDUCTOR SIZE
EMI LT1976 Input Range LT1976 Output ripple current is determined by the input to output Output Ripple LT1977 voltage ratio, inductor value and switch frequency. Since the switch frequency of the LT1977 is 2.5 times greater
FEEDBACK PIN FUNCTIONS
than that of the LT1976, the inductance used in the LT1977 application can be 2.5 times lower than the LT1976 while The feedback (FB) pin on the LT1977 is used to set output maintaining the same output ripple current. The lower voltage and provide several overload protection features. value used in the LT1977 application allows the use of a The first part of this section deals with selecting resistors physically smaller inductor. to set output voltage and the remaining part talks about frequency foldback and soft-start features. Please read
INPUT VOLTAGE RANGE
both parts before committing to a final design. The LT1976 and LT1977 minimum on and off times are Referring to Figure 2, the output voltage is determined by equivalent. This results in a narrower range of continuous a voltage divider from VOUT to ground which generates mode operation for the LT1977. Typical minimum and 1.25V at the FB pin. Since the output divider is a load on the maximum duty cycles are 6% to 95% for the LT1976 and output care must be taken when choosing the resistor 15% to 90% for the LT1977. Both parts will regulate up to divider values. For light load applications the resistor an input voltage of 60V but the LT1977 will transistion into values should be as large as possible to achieve peak pulse skipping/Burst Mode operation when the input efficiency in Burst Mode operation. Extremely large values voltage is above 30V for a 5V output. At outputs above 10V for resistor R1 will cause an output voltage error due to the 1977fa 9