LTC1873 Linear Technology, LTC1873 Datasheet - Page 30

• Manufacturer Part Number: LTC1873
• Description: Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1873
the actual load for the test, and switch it on and off while
watching the output. If this isn’t convenient, a current step
generator is needed. This generator needs to be able to
turn on and off in nanoseconds to simulate a typical
switching logic load, so stray inductance and long clip
leads between the LTC1873 and the transient generator
must be minimized.
Figure 17 shows an example of a simple transient genera-
tor. Be sure to use a noninductive resistor as the load
element—many power resistors use an inductive spiral
pattern and are not suitable for use here. A simple solution
is to take ten 1/4W film resistors and wire them in parallel
to get the desired value. Surface mount resistors are best.
This gives a noninductive resistive load which can dissi-
pate 2.5W continuously or 50W if pulsed with a 5% duty
cycle, enough for most LTC1873 circuits. Solder the
MOSFET and the resistor(s) as close to the output of the
LTC1873 circuit as possible and set up the signal genera-
tor to pulse at a 100Hz rate with a 5% duty cycle. This
pulses the LTC1873 with 500 s transients 10ms apart,
adequate for viewing the entire transient recovery time for
both positive and negative transitions while keeping the
load resistor cool.
30
U
U
W
GENERATOR
LTC1873
PULSE
Figure 17. Transient Load Generator
DUTY CYCLE
100Hz, 5%
0V TO 10V
U
50
EQUIVALENT
IRFZ44 OR
Changing the Output Voltage on the Fly
The voltage at side 1 of the LTC1873 can be changed on the
fly by changing the VID code while the output is enabled,
but care must be taken to avoid tripping the overvoltage
fault circuit. Stepping the voltage upwards abruptly is safe,
but stepping down quickly by more than 15% can leave the
system in a state where the output voltage is still at the old
higher level, but the feedback node is set to expect a new,
substantially lower voltage. If this condition persists for
more than 10 s, the overvoltage fault circuitry will fire and
latch off the LTC1873.
The simplest solution is to disable the fault circuit by
grounding the FAULT pin. Systems that must keep the
fault circuit active should ensure that the output voltage is
never programmed to step down by more than 15% in any
single step. The safest strategy is to step the output down
by 10% or less at a time and wait for the output to settle
to the new value before taking subsequent steps.
R
LOAD
LOCATE CLOSE
TO THE OUTPUT
V
OUT
1873 F17