LTC3626_15 Datasheet, PDF (18/28 Pages) Linear Technology – 20V, 2.5A Synchronous Monolithic Step-Down Regulator with Current and Temperature Monitoring

LTC3626
Applications Information
In some applications severe transients can be caused by
switching in loads with large (>10μF) input capacitors. The
discharged input capacitors are effectively put in parallel
with C_OUT, causing a rapid drop in V_OUT. No regulator can
deliver enough current to prevent this output droop if the
switch connecting the load has low resistance and is driven
quickly. The solution is to limit the turn-on speed of the
load switch driver. A Hot Swap™ controller is designed
specifically for this purpose and usually incorporates cur-
rent limit, short-circuit protection and soft-start functions.
Input/Output Current Monitor and Limit
The LTC3626 senses the average current through the
synchronous switch during the on state and outputs a
scaled replica of this current (which corresponds to the
regulator’s load current) to the IMON_OUT pin. A mirrored
version of this signal is modulated with the buck regula-
tor’s duty cycle to provide a scaled replica of the buck
regulator’s input current to the IMON_IN pin. The average
current at each of the monitor pins is 1/16000th the
measured average current. The output current at either
pin may be measured directly or converted to a voltage
with an external resistor.
The average input and output current monitor circuits
both use a chopping technique to achieve high accuracy.
As a result, a small periodic ripple may be seen at either
of these outputs, the average of which is the measured
value of interest. The ripple frequency will be the operating
frequency divided by 256. In addition, the average input
current is measured by modulating the duty cycle of the
average output current leading to an additional ripple at
the operating frequency. If required, a capacitor may be
placed on either output pin to reduce the magnitude of
the ripple.
The voltages at the IMON_OUT and IMON_IN pins are con-
tinuously fed to independent current limit amplifiers that
have a voltage reference of 1.2V (typical). A programmable
average current limit for either average output current or
average input current may be obtained by placing a resis-
tor, R_LIM, from the monitor pin to SGND according to the
following equation:
R_LIM
=
1.2V • 16000
I_LIM
where I_LIM is the programmed current limit.
When active, the current limit amplifiers form a feedback
loop that controls the maximum average current produced
by the LTC3626. Thus, when using the current limit fea-
ture, a compensation capacitor should be placed between
SGND and the monitor pin of interest. This capacitor,
combined with the R_LIM resistor, is intended to create a
dominant pole for compensation purposes. For most ap-
plications, a capacitor with a minimum value of 1µF will
provide adequate loop stability. However, given the wide
variation in loop parameters that depend on specific ap-
plication requirements, loop stability should be confirmed
by stepping the load current to a level that triggers the
programmed current limit. The resultant transient response
should provide a sense of the overall loop stability with-
out breaking the feedback loop. The transient response
that results from releasing the current limit should also
be checked. If the transient response waveforms exhibit
excessive ringing, indicating inadequate loop stability,
increase the compensation capacitor value until adequate
stability has been achieved.
The simple dominant pole compensation scheme dis-
cussed previously is intended to provide loop stability by
limiting the bandwidth of the current limit feedback loop.
As a result, the average current may momentarily exceed
the programmed limit until the current limit feedback loop
can respond. More advanced compensation networks may
be used to potentially reduce the loop response time but
generally require more caution and design expertise. For
example, one technique is to add a low value resistor in
3626fa
18
For more information www.linear.com/LTC3626