LT3991/LT3991-3.3/LT3991-5
9
3991fa
operaTion
The LT3991 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS flip-flop, turning on the internal power switch.
An amplifier and comparator monitor the current flowing
between the V
IN
and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
V
C
(see Block Diagram). An error amplifier measures the
output voltage through an external resistor divider tied to
the FB pin and servos the V
C
node. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp
on the V
C
node provides current limit. The V
C
node is
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
IN
or from the
BOOST pin. An external capacitor is used to generate a
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 efficient operation.
To further optimize efficiency, 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 L
T3991’
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
IN
is above 4.3V.
applicaTions inForMaTion
Achieving Ultralow Quiescent Current
To enhance efficiency 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 L
T3991 is in sleep mode increases, resulting
in much higher light load efficiency
. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7μA ideal. Therefore, to optimize the
Figure 1. Switching Frequency in Burst Mode Operation
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
LOAD CURRENT (mA)
SWITCHING FREQUENCY (kHz)
3991 F01
1000
200
400
600
800
0
0
10080604020
V
IN
= 12V
V
OUT
= 3.3V
