LTC2921 Datasheet, PDF (15/20 Pages) Linear Technology – Power Supply Tracker with Input Monitors

LTC2921/LTC2922 Series
APPLICATIO S I FOR ATIO
Before Q1 closes to connect the load, the actual supply
voltage relative to V_S1 is given by Equation 12.
∆V_SRC1
=
V_SRC1
–
V_S1
=
V_S1
•


R_X1
+
R_X1
R_Y1
+

R_Z1
(12)
After both Q1 and the internal remote sense switch have
closed, the load voltage relative to V_S1 is given by Equation
13.
∆V_L1
=
V_L1
–
V_S1
=
–I_L1
• R_Q1(ON)
•


R_FB1(ON)
R_X1


(13)
A small part of the load current will flow through the
remote sense switch. Use Equation 14 to calculate the
current, and do not exceed the switch current Absolute
Maximum Rating when choosing the value of R_X1.
I_DS1
=
I_L1
•


R_Q1(ON)
R_X1


(14)
In addition, once the remote sensing is active, the supply
voltage V_SRC1 will rise by approximately the drop across
the external FET. The effect on the monitor resistive divider
design has already been accounted for in the previous
section, Setting the Supply Monitor Levels.
V_SRC1
=
V_S1
+
I_L1
•
R_Q1(ON)
•
_1–
R_FB1(ON)
R_X1


(15)
≈ V_S1 + V_Q1(ON)
The terminals of each switch are interchangeable; choose
the connections to optimize the board layout. Ground all
unused switch pins.
Design Example
Consider the design of a three-supply monitoring system,
as shown in Figure 12, with specifications as listed in
Table␣ 1.
Table 1. Design Example Electrical Specifications
Supply Specifications
5V ± 7.5%
V_SRC0(MAX) = 5.375V
V_SRC0(MIN) = 4.625V
I_L0 = 0.8A (max)
3.3V ± 7.5%
V_SRC1(MAX) = 3.5475V
V_SRC1(MIN) = 3.0525V
I_L1 = 1.6A (max)
2.5V ± 7.5%
V_SRC2(MAX) = 2.6875V
V_SRC2(MIN) = 2.3125V
I_L2 = 0.4A (max)
External N-channel FET Drain-Source Voltage Specification
5V Supply
V_Q0(ON)(MAX) < 250mV
3.3V Supply
V_Q1(ON)(MAX) < 250mV
2.5V Supply
V_Q2(ON)(MAX) < 150mV
Timing Specification
TIMER Delay
t_DLY = 150ms (nom)
GATE Ramp Time t_RAMP = 500ms (nom)
Bias Current Specification
Monitor Resistive I_A1= 10µA (nom)
Divider Current I_A2 = 10µA (nom)
Other Requirements
• Remote Sense all 3 Load Voltages
• Tight Monitoring Levels
• Use Circuit Breaker Function
• DC/DC Converter Feedback Resistive Divider >100kΩ
The LTC2921 suits this application because the largest
supply in the system is 5V, and only three remote sense
switches are required.
Start with the design of the resistive dividers that set the
monitor levels. As the largest supply voltage, the 5V
supply must be connected to the V_CC pin; an internal
resistive divider sets that monitor level. Consult the
Electrical Characteristics table to confirm that V_SRC0(MIN)
>V_CC(MON)(MAX) and V_SRC0(MAX) <V_CC(OV)(MIN).
The bias current in the lower resistor for the 3.3V supply’s
dividers yields a standard 1% value of R_A1 = 49.9k:
R_A1
=
0.500V
10µA
=
50k
≈
49.9k
29212fa
15