LT3690 LINER [Linear Technology], LT3690 Datasheet - Page 11

• Manufacturer Part Number: LT3690
• Description: 42V, 2.5A Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8610 operates
in low ripple Burst Mode operation, which keeps the out-
put capacitor charged to the desired output voltage while
minimizing the input quiescent current and minimizing
output voltage ripple. In Burst Mode operation the LT8610
delivers single small pulses of current to the output capaci-
tor followed by sleep periods where the output power is
supplied by the output capacitor. While in sleep mode the
LT8610 consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1a) and the percentage
of time the LT8610 is in sleep mode increases, resulting in
Figure 1. SW Frequency vs Load Information in
Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
Minimum Load to Full Frequency (SYNC DC High)
100
800
700
600
500
400
300
200
100
80
40
60
20
0
0
0
5
5V
700kHz
V
V
IN
OUT
OUT
10
= 12V
= 3.3V
50
15
Burst Frequency
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
20
(1a)
(1b)
100
25
30
150
35
40
8610 F01b
8610 F01a
200
45
much higher light load efficiency than for typical convert-
ers. By maximizing the time between pulses, the converter
quiescent current approaches 2.5μA for a typical application
when there is no output load. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider must be minimized as it
appears to the output as load current.
While in Burst Mode operation the current limit of the top
switch is approximately 400mA resulting in output voltage
ripple shown in Figure 2. Increasing the output capacitance
will decrease the output ripple proportionally. As load ramps
upward from zero the switching frequency will increase
but only up to the switching frequency programmed by
the resistor at the RT pin as shown in Figure 1a. The out-
put load at which the LT8610 reaches the programmed
frequency varies based on input voltage, output voltage,
and inductor choice.
For some applications it is desirable for the LT8610 to
operate in pulse-skipping mode, offering two major differ-
ences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned to
the clock. In this mode much of the internal circuitry is
awake at all times, increasing quiescent current to several
hundred μA. Second is that full switching frequency is
reached at lower output load than in Burst Mode operation
(see Figure 1b). To enable pulse-skipping mode, the SYNC
pin is tied high either to a logic output or to the INTV
pin. When a clock is applied to the SYNC pin the LT8610
will also operate in pulse-skipping mode.
200mA/DIV
10mV/DIV
V
OUT
I
L
V
SYNC
Figure 2. Burst Mode Operation
= 0V
5μs/DIV
8610 F02
LT8610
11
8610p
CC