LTM4600EV Ver la hoja de datos (PDF) - Linear Technology
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LTM4600EV Datasheet PDF : 26 Pages
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LTM4600
applications information
decoupling capacitor should be placed directly adjacent
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 COUT is chosen with low enough
effective series resistance (ESR) to meet the output voltage
ripple and transient requirements. COUT 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.
VIN to VOUT Step-Down Ratios
There are restrictions in the maximum VIN to VOUT step
down ratio that can be achieved for a given input voltage.
These constraints are shown in VIN to VOUT Step-Down
Ratio in the the Typical Performance Characteristics sec-
tion. Note that additional thermal derating may apply. See
the Thermal Considerations and Output Current Derating
sections of this data sheet.
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-current la-
tchoff. Pulling the RUN/SS pin below 0.8V puts the LTM4600
into a low quiescent current shutdown (IQ ≤ 75µA). Re-
leasing the pin allows an internal 1.2µA current source
to charge up the timing capacitor CSS. Inside LTM4600,
there is an internal 1000pF capacitor from RUN/SS
pin to ground. If RUN/SS pin has an external capacitor
CSS_EXT to ground, the delay before starting is about:
tDELAY
=
1.5V
1.2µA
• (CSS _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.
After the controller has been started and given adequate
time to charge up the output capacitor, CSS is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8µA current then
begins discharging CSS. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
The over-current protection timer requires the soft-start
timing capacitor CSS be made large enough to guarantee
that the output is in regulation by the time CSS has reached
the 4V threshold. In general, this will depend upon the size
of the output capacitance, output voltage and load current
characteristic. A minimum external soft-start capacitor
can be estimated from:
CSS
_
EXT
+
1000pF
>
COUT • VOUT
10kV
Generally 0.1µF is more than sufficient.
For more information www.linear.com/LTM4600
4600fd
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