參數(shù)資料
型號(hào)： LTC3833EUDC#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 2000 kHz SWITCHING FREQ-MAX, PQCC20
封裝： 3 X 3 MM, LEAD FREE, PLASTIC, QFN-20
文件頁(yè)數(shù)： 13/36頁(yè)
文件大?。?/td> 513K
代理商： LTC3833EUDC#PBF
LTC3833
20
3833f
APPLICATIONS INFORMATION
VIN Undervoltage Lockout (UVLO)
The LTC3833 has two functions that help protect the con-
trollerincaseofinputundervoltageconditions.Aprecision
UVLO comparator constantly monitors the INTVCCvoltage
to ensure that an adequate gate-drive voltage is present.
The comparator enables UVLO and locks out the switch-
ing action until INTVCC rises above 4.2V. Once UVLO is
released, the comparator does not retrigger UVLO until
INTVCC falls below 3.65V. This hysteresis prevents oscil-
lations when there are disturbances on INTVCC.
Another way to detect an undervoltage condition is to
monitortheVINsupply.BecausetheRUNpinhasaprecision
turn-on voltage of 1.2V, one can use a resistor divider from
VIN to turn on the IC when VIN is high enough. The RUN pin
has bias currents that depend on the RUN voltage as well
asVINvoltage.Thesebiascurrentsshouldbetakenintoac-
count when designing the voltage divider and UVLO circuit
to prevent faulty conditions. Generally for RUN < 3V a bias
current of 1.3μA flows out of the RUN pin, and for RUN >
3V, correspondingly increasing current flows into the pin,
reaching a maximum of about 35μA for RUN = 6V.
Soft-Start and Tracking
The LTC3833 has the ability to either soft-start by itself
with a capacitor or track the output of an external supply.
Soft-start or tracking features are achieved not by limiting
the maximum output current of the switching regulator
but by controlling the regulator’s output voltage according
to the ramp rate on the TRACK/SS pin.
When configured to soft-start by itself, a capacitor should
be connected to the TRACK/SS pin. TRACK/SS is pulled
low until the RUN pin voltage exceeds 1.2V and UVLO is
released, at which point an internal current of 1μA charges
the soft-start capacitor, CSS, connected to TRACK/SS.
Current foldback is disabled during this phase to ensure
smooth soft-start or tracking. The soft-start or tracking
range is defined to be the voltage range from 0V to 0.6V
on the TRACK/SS pin. The total soft-start time can be
calculated as:
tSOFTSTART = 0.6V
CSS
1A
When the LTC3833 is configured to track another supply,
a voltage divider can be used from the tracking supply to
the TRACK/SS pin to scale the ramp rate appropriately.
Two common implementations of tracking as shown in
Figure 6a are coincident and ratiometric. For coincident
tracking, make the divider ratio from the external supply
the same as the divider ratio for the differential feedback
voltage. Ratiometric tracking could be achieved by using
a different ratio than the differential feedback (Figure 6b).
Note that the small soft-start capacitor charging current is
alwaysflowing,producingasmalloffseterror.Tominimize
this error, select the tracking resistive divider values to be
small enough to make this offset error negligible.
Phase and Frequency Synchronization
For applications that require better control of EMI and
switchingnoiseorhavespecialsynchronizationneeds,the
LTC3833canphaseandfrequencysynchronizetheturn-on
of the top MOSFET to an external clock signal applied to
the MODE/PLLIN pin. The applied clock signal needs to
be within ±30% of the RT pin programmed free-running
frequency to assure proper frequency and phase lock.
The clock signal levels should generally comply to VIH >
2V and VIL < 0.5V. The MODE/PLLIN pin has an internal
600k pull-down resistor to ensure pulse-skipping mode
if the pin is left floating.
The LTC3833 uses the voltages on VIN and VOUT pins as
well as the RT programmed frequency to determine the
steady-state on-time as follows:
tON ≈
VOUT
VIN f
An internal PLL system adjusts this on-time dynamically
in order to maintain phase and frequency lock with the
external clock. The LTC3833 will maintain phase and fre-
quency lock under steady-state conditions for VIN, VOUT
and load current.
As shown in the previous equation, the top MOSFET on-
time is a function of the switching regulator’s output. This
output is measured by the VOUTpinandisusedtocalculate
the required on-time. Therefore, simply connecting VOUT
to the regulator’s local output point is preferable for most
applications. However, there could be applications where
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