LTM4600_06 Datasheet, PDF (10/24 Pages) Linear Technology – 10A High Effi ciency DC/DC μModule

LTM4600
APPLICATIONS INFORMATION
the module input pins in the PCB layout to minimize the
trace inductance and high frequency AC noise.
Output Capacitors
The LTM4600 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 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.
Fault Conditions: Current Limit and Over current
Foldback
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.
To further limit current in the event of an over load condi-
tion, the LTM4600 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
LTM4600 as well as a timer for soft-start and over-cur-
rent latchoff. Pulling the RUN/SS pin below 0.8V puts
the LTM4600 into a low quiescent current shutdown (I_Q
≤ 75μA). Releasing the pin allows an internal 1.2μA cur-
rent source to charge up the timing capacitor C_SS. Inside
LTM4600, 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_DELAY
=
1.5V
1.2μA
•
(C_{SS _ EXT}
+
1000pF)
When the voltage on RUN/SS pin reaches 1.5V, the LTM4600
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 contraints 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.
After the controller has been started and given adequate
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