Datasheet

LTM4602
10
4602fa
Output Capacitors
The LTM4602 is designed for low output voltage ripple. The
bulk output capacitors C
OUT
is chosen with low enough
effective series resistance (ESR) to meet the output voltage
ripple and transient requirements. C
OUT
can be low ESR
tantalum capacitor, low ESR polymer capacitor or ceramic
capacitor (X5R or X7R). The typical capacitance is 200μF
if all ceramic output capacitors are used. The internally
optimized loop compensation provides suffi cient stability
margin for all ceramic capacitors applications. Additional
output fi ltering may be required by the system designer,
if further reduction of output ripple or dynamic transient
spike is required. Refer to Table 2 for an output capaci-
tance matrix for each output voltage droop, peak to peak
deviation and recovery time during a 3A/μs transient with
a specifi c output capacitance.
Fault Conditions: Current Limit and Overcurrent
Foldback
The LTM4602 has a current mode controller, which inher-
ently limits the cycle-by-cycle inductor current not only in
steady-state operation, but also in transient.
To further limit current in the event of an over load condi-
tion, the LTM4602 provides foldback current limiting. If the
output voltage falls by more than 50%, then the maximum
output current is progressively lowered to about one sixth
of its full current limit value.
Soft-Start and Latchoff with the RUN/SS pin
The RUN/SS pin provides a means to shut down the
LTM4602 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTM4602 into a low quiescent current shutdown (I
Q
≤
100μA). Releasing the pin allows an internal 1.2μA cur-
rent source to charge up the timing capacitor C
SS
. Inside
LTM4602, there is an internal 1000pF capacitor from RUN/
SS pin to ground. If RUN/SS pin has an external capacitor
C
SS_EXT
to ground, the delay before starting is about:
t
D ELAY
=
1.5V
1.2μA
•(C
SS_EXT
+ 1000pF)
When the voltage on RUN/SS pin reaches 1.5V, the LTM4602
internal switches are operating with a clamping of the
maximum output inductor current limited by the RUN/SS
pin total soft-start capacitance. As the RUN/SS pin voltage
rises to 3V, the soft-start clamping of the inductor current
is released.
V
IN
to V
OUT
Step-Down Ratios
There are restrictions in the maximum V
IN
to V
OUT
step
down ratio that can be achieved for a given input voltage.
These constraints are shown in the Typical Performance
Characteristics curves labeled “V
IN
to V
OUT
Step-Down
Ratio”. Note that additional thermal derating may apply. See
the Thermal Considerations and Output Current Derating
sections of this data sheet.
APPLICATIONS INFORMATION