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LM1771UMM Datenblatt(PDF) 11 Page - Texas Instruments

Teilenummer LM1771UMM
Bauteilbeschribung  Low-Voltage Synchronous Buck Controller
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LM1771
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SNVS446D – JUNE 2006 – REVISED JANUARY 2016
Table 2. Frequency vs VOUT for Variants
TIMING OPTIONS (1)
VOUT
500 ns
1000 ns
2000 ns
0.8
485
242
121
1
606
303
152
1.2
727
364
182
1.5
909
455
227
1.8
1091
545
273
2.5
1515
758
379
3.3
2000
1000
500
(1)
Switching Frequency (kHz) of LM1771 based on output voltage and
timing option.
7.3.2 Short-Circuit Protection
The LM1771 has an internal short-circuit comparator that constantly monitors the feedback node (except during
soft-start). If the feedback voltage drops below 0.55 V (equivalent to the output voltage dropping below 68% of
nominal), the comparator trips causing the part to latch off. The LM1771 does not resume switching until the
input voltage is taken below the UVLO threshold and then brought back into its normal operating range, or the
part is disabled then re-enabled through the enable pin. The purpose of this function is to prevent a severe short
circuit from causing damage to the application. Due to the fast transient response of the LM1771 a severe short
on the output causing the feedback to drop would only occur if the load applied had an effective resistance that
approaches the PMOS RDS(ON).
7.3.3 Precision Enable
The LM1771 features a precision enable circuit. If the voltage on the EN pin is 1.2 V or greater, the part is
enabled and switching occurs. If the enable voltage falls below 1.2 V, the part is placed into a shutdown state
and the drivers is tri-stated. This allows the LM1771 to be easily sequenced using a resistive divider from the
output of another regulator, or the working input voltage range of the LM1771 to be set using a resistive divider
on VIN. There is no internal pullup connected to the EN pin, so an external signal is required to initiate switching.
NOTE
When power is first applied to the LM1771, there is a slight delay before the enable
comparator is functional. During this delay, typically on the order of 400 µs, the part is
disabled regardless of the voltage on the EN pin. The falling enable threshold features 50
mV of hysteresis
7.3.4 Soft-Start
To limit in-rush current and allow for a controlled start-up the LM1771 incorporates an internal soft-start scheme.
Every time the enable voltage rises above 1.2 V while VIN is greater than the UVLO threshold, the LM1771 goes
through an adaptive soft-start that limits the ON-time and expands the minimum OFF-time. In addition the part
only activates the PMOS allowing a discontinuous mode of operation enabling a prebiased start-up. The time
spent in soft-start depends on the load applied to the output, but is usually close to a set time that is dependent
on the timing option. The approximate soft-start time can be seen in Table 3 for each timing option.
Table 3. Soft-Start Timing for All Variants
PRODUCT ID
TIMING
TSS
LM1771S
0.5 µs
1 ms
LM1771T
1.0 µs
1.2 ms
LM1771U
2.0 µs
1.8 ms
Copyright © 2006–2016, Texas Instruments Incorporated
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