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