Datasheet
LTC4215-1/LTC4215-3
17
421513fc
Current Limit Stability
For many applications the LTC4215-1/LTC4215-3 current
limits will be stable without additional components. How-
ever there are certain conditions where additional compo-
nents may be needed to improve stability. The dominant
pole of the current limit circuit is set by the capacitance
and resistance at the gate of the external MOSFET, and
larger gate capacitance makes the current limit loop more
stable. Usually a total of 8nF gate to source capacitance
is suffi cient for stability and is typically provided by in-
herent MOSFET C
GS
, however the stability of the loop is
degraded by increasing R
SENSE
or by reducing the size of
the resistor on a gate RC network if one is used, which
may require additional gate to source capacitance. Board
level short-circuit testing is highly recommended as board
layout can also affect transient performance, for stability
testing the worst case condition for current limit stabil-
ity occurs when the output is shorted to ground after a
normal start-up.
There are two possible parasitic oscillations when the
MOSFET operates as a source follower when ramping
at power-up or during current limiting. The fi rst type of
oscillation occurs at high frequencies, typically above
1MHz. This high frequency oscillation is easily damped
with R5 as shown in Figure 1. In some applications, one
may fi nd that R5 helps in short-circuit transient recovery
as well. However, too large of an R5 value will slow down
the turn-off time. The recommended R5 range is between
5 and 500.
The second type of source follower oscillation occurs
at frequencies between 200kHz and 800kHz due to load
capacitance being between 0.2µF and 9µF, the presence
of R5 resistance, the absence of a drain bypass capacitor,
a combination of bus wiring inductance and bus supply
o u t p u t i m p e d a n c e . To p r e v e n t t h e s e c o n d t y p e o f o s c i l l a t i o n
avoid load capacitance below 10µF, alternately connect an
external capacitor from the MOSFET gate to ground with
a value greater than 1.5nF.
Supply Transients
The LTC4215-1/LTC4215-3 are designed to ride through
s u p p l y t r a n s i e n t s c a u s e d b y l o a d s t e p s . I f t h e r e i s a s h o r t e d
load and the parasitic inductance back to the supply is
greater than 0.5µH, there is a chance that the supply col-
lapses before the active current limit circuit brings down
the GATE pin. If this occurs, the undervoltage monitors
pull the GATE pin low. The undervoltage lockout circuit
has a 2µs fi lter time after V
DD
drops below 2.74V. The UV
pin reacts in 2µs to shut the GATE off, but it is recom-
mended to add a fi lter capacitor C
F
to prevent unwanted
shutdown caused by a transient. Eventually either the UV
pin or undervoltage lockout responds to bring the current
under control before the supply completely collapses.
Supply Transient Protection
The LTC4215-1/LTC4215-3 are safe from damage with
supply voltages up to 24V. However, spikes above 24V
may damage the part. During a short-circuit condition,
Figure 5. Recommended Layout
APPLICATIONS INFORMATION
UV
OV
SS
GND
ON
EN
SDAO
FB
GPIO1
INTV
CC
TIMER
ADIN
GPIO3
ADR1
V
DD
SENSE
+
SENSE
–
GATE
SOURCE
SDAI
SCL
GPIO2
NC
ADR0
R2
R3
C
F
Z1
R1
SENSE RESISTOR R
S
C3
LTC4215-1
R8
I
LOAD
4215 F05
I
LOAD