English
Language : 

LTC3716 Datasheet, PDF (18/28 Pages) Linear Technology – 2-Phase, 5-Bit VID, Current Mode, High Efficiency, Synchronous Step-Down Switching Regulator
◁
LTC3716
APPLICATIO S I FOR ATIO
Table 1. VID Output Voltage Programming
VID4
VID3
VID2 VID1 VID0
0
0
0
0
0
0
0
0
0
1
0
0
0
1
0
0
0
0
1
1
0
0
1
0
0
0
0
1
0
1
0
0
1
1
0
0
0
1
1
1
0
1
0
0
0
0
1
0
0
1
0
1
0
1
0
0
1
0
1
1
0
1
1
0
0
0
1
1
0
1
0
1
1
1
0
0
1
1
1
1
1
0
0
0
0
1
0
0
0
1
1
0
0
1
0
1
0
0
1
1
1
0
1
0
0
1
0
1
0
1
1
0
1
1
0
1
0
1
1
1
1
1
0
0
0
1
1
0
0
1
1
1
0
1
0
1
1
0
1
1
1
1
1
0
0
1
1
1
0
1
1
1
1
1
0
1
1
1
1
1
LTC3716
1.750V
1.700V
1.650V
1.600V
1.550V
1.500V
1.450V
1.400V
1.350V
1.300V
1.250V
1.200V
1.150V
1.100V
1.050V
1.000V
0.975V
0.950V
0.925V
0.900V
0.875V
0.850V
0.825V
0.800V
0.775V
0.750V
0.725V
0.700V
0.675V
0.650V
0.625V
0.600V
Soft-Start/Run Function
The RUN/SS pin provides three functions: 1) Run/Shut-
down, 2) soft-start and 3) a defeatable short-circuit latchoff
timer. Soft-start reduces the input power sources’ surge
currents by gradually increasing the controller’s current
limit ITH(MAX). The latchoff timer prevents very short,
extreme load transients from tripping the overcurrent
latch. A small pull-up current (>5µA) supplied to the RUN/
SS pin will prevent the overcurrent latch from operating.
The following explanation describes how the functions
operate.
An internal 1.2µA current source charges up the soft-start
capacitor, CSS. When the voltage on RUN/SS reaches 1.5V,
the controller is permitted to start operating. As the voltage
on RUN/SS increases from 1.5V to 3.0V, the internal
current limit is increased from 25mV/RSENSE to 75mV/
RSENSE. The output current limit ramps up slowly, taking
an additional 1.4s/µF to reach full current. The output
current thus ramps up slowly, reducing the starting surge
current required from the input power supply. If RUN/SS
has been pulled all the way to ground there is a delay before
starting of approximately:
( ) tDELAY
=
1.5V
1.2µA
CSS
=
1.25s/µF
CSS
The time for the output current to ramp up is then:
( ) tIRAMP
=
3V − 1.5V
1.2µA
CSS
=
1.25s/µF
CSS
By pulling the RUN/SS pin below 0.8V the LTC3716 is put
into low current shutdown (IQ < 40µA). The RUN/SS pins
can be driven directly from logic as shown in Figure 6.
Diode D1 in Figure 6 reduces the start delay but allows
CSS to ramp up slowly providing the soft-start function.
The RUN/SS pin has an internal 6V zener clamp (see
Functional Diagram).
VIN
3.3V OR 5V
RUN/SS
RSS*
D1
CSS
INTVCC
RSS*
RUN/SS
D1*
CSS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
3716 F06
Figure 6. RUN/SS Pin Interfacing
18
 ▷