LTC3862 Datasheet, PDF (16/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
OPERATION
Operation of the RUN Pin
The control circuitry in the LTC3862 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:
I_IN
=
I_OUT •
V_IN
V_OUT
•η
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 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:
V_IN(ON)
=
1.22V
⎛
⎝⎜ ¹⁺
R_A
R_B
⎞
⎠⎟
–
0.5μ
•
R_A
V_IN(OFF)
=
⎛
1.22V _⎝⎜ 1+
R_A
R_B
⎞
⎠⎟
–
5μ
•
R_A
Several of the possible RUN pin control techniques are
illustrated in Figure 5.
Frequency Selection and the Phase Lock Loop
The selection of the switching frequency is a tradeoff
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.
16
V_IN
INTERNAL 5V
LTC3862
0.5μA
RUN
10V
SGND
4.5μA
+
1.22V _–
BIAS AND
START-UP
CONTROL
RUN
COMPARATOR
3862 F05a
Figure 5a. Using the RUN Pin for a “Normally On” Converter
V_IN
INTERNAL 5V
EXTERNAL
LOGIC
CONTROL ^0.5μA
RUN
4.5μA
+
10V
SGND
1.22V _–
LTC3862
BIAS AND
START-UP
CONTROL
RUN
COMPARATOR
3862 F05b
Figure 5b. On/Off Control Using External Logic
V_IN
INTERNAL 5V
R_A
0.5μA
RUN
4.5μA
+
R_B 10V
SGND
1.22V _–
LTC3862
BIAS AND
START-UP
CONTROL
RUN
COMPARATOR
3862 F05c
Figure 5c. Programming the Input Voltage Turn-On and Turn-Off
Thresholds Using the RUN Pin
3862fb