Linear Technology DC2641A, Демонстрационное руководство, Страница: 4 / 10

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dc2641af
DEMO MANUAL DC2641A
FREQUENCY SYNCHRONIZATION AND MODE SELECTION
Table 1. Mode Selection and Synchronized Operation Options
CONFIGURATION JP4 MODE WITH SYNC. SIGNAL APPLIED TO PLLIN
Forced Continuous Operation FCM FCM
Pulse-Skipping Operation P.S. P.S.
Burst Mode Operation with Adjustable Clamp Level Burst ADJ. FCM
Burst Mode Operation with Default Clamp Burst Default FCM
Table 2. Optional Inductor DCR Current Sensing
CONFIGURATION RS1 R24 R28 C16 R37 R38 R39 R40
Current Sense Resistor (Default) Ref. Sch. Ref. Sch. Ref. Sch. OPEN OPEN OPEN OPEN OPEN
Inductor DCR Current Sensing 0Ω Copper OPEN OPEN Calculated Value from Data Sheet 0Ω 0Ω
Demonstration circuit 2641A’s Mode selector allows the
converter to run in forced continuous operation, pulse-
skipping operation, Burst Mode or Burst mode with
adjustable clamp level by changing the position of JP4.
To synchronize the DC2641A to an external clock, apply
the sync signal to the PLLIN turret. Depending upon the
JP4 setting, the DC2641A will operate in different modes.
See Table 1 for the detailed description.
100% DUTY
Demonstration circuit 2641A features 100% duty cycle
operation in dropout. Set the JP5 to “ON” position to acti-
vate this feature. Set JP5 to “OFF” position disables the
internal charge pump and enables boost refresh, allowing
for 99% duty cycle operation in dropout.
OPTIONAL INDUCTOR DCR CURRENT SENSING
Demonstration circuit 2641A provides an optional circuit
for Inductor DCR Current Sensing. Inductor DCR Cur-
rent Sensing uses the DCR of the inductor to sense the
inductor current instead of discrete sense resistors. The
advantages of DCR sensing are lower cost, reduced board
space and higher efficiency, but the disadvantage is a less
accurate current limit. If DCR sensing is used, be sure to
select an inductor current with a sufficiently high satura-
tion current or use an iron powder type material.
Refer to Table 2 for Optional Inductor DCR Current Sens-
ing setup and to the data sheet for more details.
Low Quiescent Current Applications and Measurement
The typical quiescent current (I
Q
) of the LTC7801 control-
ler is 40µA in sleep mode as specified in the LTC7801
data sheet. However, the input current of the DC2641A
board can be higher than this value because of additional
circuit outside of the IC. To reduce the total input current,
large value FB divider resistors should be used. In addi-
tion, some jumpers and resistors should be configured
accordingly. Refer to Table 3 for the low input quiescent
current setup.
Table 3. Low Input Quiescent Current Configuration
REFERENCE DESIGNATOR R1 R2 R23 JP3 JP4 JP5 JP8
Function OVLO PGOOD INTV
CC
Jumper MODE Selector 100% DUTY BIAS supply
Stuffing Option OPEN 0Ω OPEN OPEN BURST ADJ. or
BURST DEFAULT
OFF OFF