LTC2921CGN データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LTC2921CGN

Power Supply Tracker with Input Monitors

Linear Technology 

LTC2921/LTC2922 Series

APPLICATIO S I FOR ATIO

VSRC0

RSENSE

Q0

VL0

VCC SENSE

GATE

VPUMP

PG

ENABLE

4µA

PG

RG0

10Ω

CGATE

RPG

µC

RESET

LTC2922

GND

2921/22 F08

Figure 8. PG Pin as Logic Output

VSRC5

Q5

VL5

VSRC0

RSENSE

RG5

Q0

10Ω

VL0

VCC SENSE

GATE

VPUMP

4µA

PG

PG

ENABLE

RG0

10Ω

CGATE

CPG

LTC2922

GND

2921/22 F09

Figure 9. PG Pin as Sequenced N-Channel Gate Driver

pin has fully ramped up. The switches are N-channel

MOSFETs whose gates are ramped from ground to VPUMP

at a nominal rate of 8V/ms. The PG pin is activated upon

completion of the TIMER delay cycle that follows GATE

ramp-up and remote sense switch activation. When con-

ditions indicate a supply disconnect, the switches shut off

in less than 10µs.

Figure 10 shows an example of how to connect a switch to

remote sense the load voltage. Although only one remote

sense switch is referred to in this section, the calculations

and comments apply to all.

Before the activation of Q1 and the internal switch, resistor

RX1 provides a direct path between the DC/DC converter’s

output voltage and its feedback network (RY1 and RZ1).

Once Q1 activates, the supply energizes the load. When

the internal switch turns on, it provides a remote sense

path between the load voltage and the converter’s feed-

back network.

14

VOUT

VFB

GND

DC/DC

CONVERTER

VSRC1

RX1 RB1

RY1

RZ1

RA1

VS1

Q1

VL1

V1

GATE

LTC2922

RG1

10Ω

CGATE

S1

D1

GND

LOAD

2921/22 F10

Figure 10. Automatic Remote Sense Switching Connection

To choose a value for resistor RX1, consider the remote

sense switch connection equivalent network in Figure 11.

Resistor RQ1(ON) represents the on-resistance of Q1, and

resistor RFB1(ON) represents the on-resistance of the inter-

nal switch.

VSRC1

IQ1

VOUT

VFB

RZ1

GND

RX1

RY1

IDS1

VS1

RQ1(ON)

LTC2922

S1

D1

RFB1(ON)

VL1

LOAD

IL1

DC/DC

CONVERTER

2921/22 F11

Figure 11. Remote Sense Switch Connection Equivalent Network

To allow the load voltage to dominate the feedback to the

converter when the internal switch is closed, make RX1 >>

RFB1(ON). To set the converter feedback ratio accurately

with RY1 and RZ1, make both RX1 and RFB1(ON) much less

than (RY1 + RZ1). To ensure that most of the load current

flows through the external N-channel FET, choose (RX1 +

RFB1(ON)) >> RQ1(ON). Summarized, these requirements

amount to:

RQ1(ON), RFB(ON) << RX1 << (RY1 + RZ1)

(11)

Approach the selection of RX1 in the following way: design

the DC/DC converter feedback based on the resistive

divider formed by RY1 and RZ1 with VS1 at the desired

supply voltage value. When the resistor values satisfy

Equation 11, Equations 12 through 15 are valid.

29212fa

Share Link: