LTM4600
APPLICATIONS INFORMATION
the module input pins in the PCB layout to minimize the
trace inductance and high frequency AC noise.
Soft-Start and Latchoff with the RUN/SS pin
The RUN/SS pin provides a means to shut down the
LTM4600 as well as a timer for soft-start and over-cur-
rent latchoff. Pulling the RUN/SS pin below 0.8V puts
Output Capacitors
TheLTM4600isdesignedforlowoutputvoltageripple.The
the LTM4600 into a low quiescent current shutdown (I
Q
bulk output capacitors C
is chosen with low enough
≤ 75μA). Releasing the pin allows an internal 1.2μA cur-
OUT
effectiveseriesresistance(ESR)tomeettheoutputvoltage
ripple and transient requirements. C can be low ESR
rent source to charge up the timing capacitor C . Inside
SS
LTM4600, thereisaninternal1000pFcapacitorfromRUN/
OUT
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 sufficient stability
margin for all ceramic capacitors applications. Additional
output filtering 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 5A/μs transient with
a specific output capacitance.
SS pin to ground. If RUN/SS pin has an external capacitor
C
to ground, the delay before starting is about:
SS_EXT
1.5V
1.2μA
tDELAY
=
•(CSS_EXT +1000pF)
WhenthevoltageonRUN/SSpinreaches1.5V,theLTM4600
internal switches are operating with a clamping of the
maximum output inductor current limited by the RUN/SS
pintotalsoft-startcapacitance. AstheRUN/SSpinvoltage
rises to 3V, the soft-start clamping of the inductor current
is released.
Fault Conditions: Current Limit and Over current
Foldback
V to V
Step-Down Ratios
IN
OUT
There are restrictions in the maximum V to V
step
IN
OUT
The LTM4600 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.
down ratio that can be achieved for a given input voltage.
These contraints are shown in the Typical Performance
Characteristics curves labeled “V to V
Step-Down
IN
OUT
To further limit current in the event of an over load condi-
tion,theLTM4600providesfoldbackcurrentlimiting.Ifthe
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.
Ratio”.Notethatadditionalthermalderatingmayapply.See
the Thermal Considerations and Output Current Derating
sections of this data sheet.
After the controller has been started and given adequate
4600fc
10
Linear [ Linear ]
