6
dc2194af
DEMO MANUAL DC2194A
Quick start proceDure
8. To program other output voltages for Channel 1 or
Channel 2, insert correct values of the bottom feedback
resistors (Table 1).
These values are calculated based on a typical feedback
reference voltage of 0.6V and fixed internal top feedback
resistor of 60.4kΩ.
Table 1. Bottom Resistive Divider Values (1%) for Setting
Typical Output Voltages
V
OUT
(V)
1.0
1.2
1.5
1.8
2.5
3.3
5.0
R
BOT
(kΩ)
90.9
60.4
40.2
30.1
19.1
13.3
8.25
* NOTE 2: LTM4642 has been internally compensated
for most of input, output voltages and frequency ranges.
However, to obtain the best efficiency, thermal and load
transient response performance when selecting output
voltages different than the demo board default set voltages,
the following parameters need to be optimized accordingly:
input voltages, switching frequency, output capacitors
and optional external compensation values (Feedforward
Capacitors: C1, C12 and C
COMP
: C3, C11). Please refer to
Table 2 for more details.
Table 2. Suggested Optimized Switching Frequency for Typical
Input and Output Voltages
V
IN
(V)
3.3V 5.0V
5V
12V 12V 12V 20V 20V 20V
V
OUT
(V)
1V
1.2V
1.5V
1.8V
1V
1.2V
1.5V
1.8V
1.8V
2.5V
3.3V
1.0V
1.2V
1.5V
1.8V
2.5V
2.5V
3.3V
5.0V
1V
1.2V
1.5V
1.8V
2.5V
3.3V
5.0V
f
SW
(kHz)
600
650
800
650
800 1000
1200
650
800 1000
1200
R5
(kΩ)
R_freq
68.1 66.5 49.9 66.5 49.9 39.2
32.4
66.5 49.9 39.2
32.4
Differential Output Voltage Sensing
The LTM4642 includes an internal low offset, high input
impedance, unity gain, high bandwidth differential ampli-
fier for applications that require true remote sensing. This
feature allows users to accurately sense the output voltage
across the output capacitor at the load point in a widely
distributed power system where power trace’s parasitic
voltage drops are always presented. The differential ampli-
fier’s output is internally connected to the error amplifier’s
inverting input. V
OUTS1
+ and V
OUTS1
– are Kelvin connected
directly across C
OUT1
on DC2194A.
(Optional) Output Voltage Tracking
TRK/SS1 and TRK/SS2 allow users to program output
voltage supply tracking during start-up or shutdown while
operating several voltage supply rails at the same time.
Channel 1 is configured as a master and Channel 2 is a
slave channel on DC2194A. Coincident tracking mode
can be implemented by connecting TRK/SS2 of the
slave channel to the mid-point of an additional resistive
divider (R19, R20) to the master channel’s output volt-
age. The ratio of this divider is identical to that of the
slave’s channel feedback divider. In this tracking mode,
output voltage of the master channel must be higher
than the output voltage of the slave channel. The rising
time of the output voltage can be adjusted by changing
the soft start capacitor’s values of the master channel.
Coincident tracking mode can be activated by inserting
JP6 between Pin 1 and 2 of TRACK2 SEL and perform-
ing start-up/shutdown by releasing RUN1 from GND
and pulling RUN1 to GND accordingly. Tracking mode
can be observed by monitoring V
OUT1
, V
OUT2
and FB1,
FB2 using scope probes. The same method can be used
to configure V
OUT1
or V
OUT2
tracking an external supply
voltage by inserting JP3 (TRK1 SEL) to TRACK or JP6
(TRK2 SEL) to EXT, applying an external voltage supply
at TRACK1 (E8) or TRACK2 (E13) and repeating start-up/
shutdown test to evaluate the tracking function of the
regulator. Ratiometric tracking mode can be achieved by
connecting TRK/SS2 to FB1. Ratiometric tracking mode
can save two resistors while coincident tracking mode
offers better voltage regulation. It is optional for users
to determine the most appropriate tracking method for
the power supply design.
(Optional) External Frequency Synchronization
The MODE/PLLIN pin can be used to synchronize the
internal oscillator clock frequency to the external clock
signal. Place JP2 (MODE/PLLIN) to CLKIN, apply an
external clock at CLKIN (E10) to vary the switching fre-
quency within ±30% of the set frequency. The external
clock input high threshold is 2V typical, while the input
low threshold is 0.5V.
