Linear Technology DC2641A Demo Manual page 4

DEMO MANUAL DC2641A
FREQUENCY SYNCHRONIZATION AND MODE SELECTION
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-
Table 1. Mode Selection and Synchronized Operation Options
CONFIGURATION
Forced Continuous Operation
Pulse-Skipping Operation
Burst Mode Operation with Adjustable Clamp Level
Burst Mode Operation with Default Clamp
Table 2. Optional Inductor DCR Current Sensing
CONFIGURATION
Current Sense Resistor (Default)
Inductor DCR Current Sensing
Table 3. Low Input Quiescent Current Configuration
REFERENCE DESIGNATOR
Function
Stuffing Option
4
Burst Default
RS1 R24
Ref. Sch. Ref. Sch. Ref. Sch.
0Ω Copper OPEN
R1 R2 R23
OVLO PGOOD
OPEN 0Ω OPEN
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
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.
JP4 MODE WITH SYNC. SIGNAL APPLIED TO PLLIN
FCM
P .S.
Burst ADJ.
R28 C16 R37
OPEN OPEN
OPEN Calculated Value from Data Sheet
JP3
INTV Jumper
CC
OPEN
) of the LTC7801 control-
Q
FCM
P .S.
FCM
FCM
R38 R39
OPEN OPEN
0Ω
JP4 JP5
MODE Selector 100% DUTY
BURST ADJ. or OFF
BURST DEFAULT
R40
OPEN
0Ω
JP8
BIAS supply
OFF
dc2641af