lt3991 Linear Technology Corporation, lt3991 Datasheet - Page 9

• Manufacturer Part Number: lt3991
• Description: 55v, 1.2a Step-down Regulator With 2.8?a Quiescent Current
• Manufacturer: Linear Technology Corporation

OPERATION
The LT3991 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS fl ip-fl op, turning on the internal power switch.
An amplifi er and comparator monitor the current fl owing
between the V
this current reaches a level determined by the voltage at
V
output voltage through an external resistor divider tied to
the FB pin and servos the V
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp
on the V
also clamped by the voltage on the SS pin; soft-start is
implemented by generating a voltage ramp at the SS pin
using an external capacitor and resistor.
If the EN pin is low, the LT3991 is shut down and draws
700nA from the input. When the EN pin exceeds 1.01V,
the switching regulator will become active.
The switch driver operates from either V
BOOST pin. An external capacitor is used to generate a
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance effi ciency at light loads, the LT3991 operates
in low ripple Burst Mode, which keeps the output capacitor
charged to the desired output voltage while minimizing
the input quiescent current. In Burst Mode operation the
LT3991 delivers single pulses of current to the output ca-
pacitor followed by sleep periods where the output power
is supplied by the output capacitor. When in sleep mode
the LT3991 consumes 1.7μA, but when it turns on all the
circuitry to deliver a current pulse, the LT3991 consumes
1.5mA of input current in addition to the switch current.
Therefore, the total quiescent current will be greater than
1.7μA when regulating.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3991 is in sleep mode increases, resulting
in much higher light load effi ciency. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7μA ideal. Therefore, to optimize the
C
(see Block Diagram). An error amplifi er measures the
C
node provides current limit. The V
IN
and SW pins, turning the switch off when
C
node. If the error amplifi er’s
IN
or from the
C
node is
voltage at the BOOST pin that is higher than the input
supply. This allows the driver to fully saturate the internal
bipolar NPN power switch for effi cient operation.
To further optimize effi ciency, the LT3991 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input supply
current to 1.7μA. In a typical application, 2.8μA will be con-
sumed from the supply when regulating with no load.
The oscillator reduces the LT3991’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during start-
up and overload.
The LT3991 contains a power good comparator which
trips when the FB pin is at 91% of its regulated value. The
PG output is an open-drain transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3991 is
enabled and V
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
Figure 1. Switching Frequency in Burst Mode Operation
1000
800
600
400
200
0
0
FRONT PAGE APPLICATION
V
V
IN
IN
OUT
= 12V
is above 4.3V.
= 3.3V
20
LOAD CURRENT (mA)
40
60
80
100
3991 F01
120
LT3991
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