LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 17

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

Operation of the RUN Pin
The control circuitry in the LTC3862-2 is turned on and
off using the RUN pin. Pulling the RUN pin below 1.22V
forces shutdown mode and releasing it allows a 0.5μA
current source to pull this pin up, allowing a “normally
on” converter to be designed. Alternatively, the RUN pin
can be externally pulled up or driven directly by logic.
Care must be taken not to exceed the absolute maximum
rating of 8V for this pin.
The comparator on the RUN pin can also be used to sense
the input voltage, allowing an undervoltage detection
circuit to be designed. This is helpful in boost converter
applications where the input current can reach very high
levels at low input voltage:
The 1.22V input threshold of the RUN comparator is derived
from a precise bandgap reference, in order to maximize
the accuracy of the undervoltage-sensing function. The
RUN comparator has 80mV built-in hysteresis. When the
voltage on the RUN pin exceeds 1.22V, the current sourced
into the RUN pin is switched from 0.5μA to 5μA PTAT
(proportional to absolute temperature) current. The user
can therefore program both the rising threshold and the
amount of hysteresis using the values of the resistors in
the external divider, as shown in the following equations:
Several of the possible RUN pin control techniques are
illustrated in Figure 5.
Frequency Selection and the Phase-Locked Loop
The selection of the switching frequency is a trade-off
between efficiency and component size. Low frequency
operation increases efficiency by reducing MOSFET
switching losses, but requires a larger inductor and output
capacitor to maintain low output ripple.
operaTion
I
V
V
IN
IN(ON)
IN(OFF)
=
I
OUT
= 1.22V 1+
= 1.22V 1+
V
• V
IN
OUT

 

 
R
R
R
R
B
A
A
B

 

 
– 0.5µ • R
– 5µ • R
A
A
Figure 5c. Programming the Input Voltage Turn-On and Turn-Off
Thresholds Using the RUN Pin
Figure 5a. Using the RUN Pin for a “Normally On” Converter
EXTERNAL
CONTROL
LOGIC
R
R
Figure 5b. On/Off Control Using External Logic
A
B
0.5µA
0.5µA
0.5µA
V
RUN
SGND
V
RUN
SGND
V
RUN
SGND
10V
10V
10V
IN
IN
IN
INTERNAL 5V
INTERNAL 5V
INTERNAL 5V
1.22V
1.22V
1.22V
4.5µA
4.5µA
4.5µA
–
–
–
+
+
+
RUN
COMPARATOR
RUN
COMPARATOR
RUN
COMPARATOR
LTC3862-2
BIAS AND
START-UP
CONTROL
BIAS AND
START-UP
CONTROL
BIAS AND
START-UP
CONTROL
LTC3862-2
LTC3862-2
LTC3862-2
38622 F05a
38622 F05b
38622 F05c
17
38622f