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LM2695SDX 数据表(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部件名 LM2695SDX
功能描述  High Voltage (30V, 1.25A) Step Down Switching Regulator
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制造商  NSC [National Semiconductor (TI)]
网页  http://www.national.com
标志 NSC - National Semiconductor (TI)

LM2695SDX 数据表(HTML) 10 Page - National Semiconductor (TI)

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ON-Time Timer, and Shutdown
(Continued)
The LM2695 can be remotely shut down by taking the
RON/SD pin below 0.8V. See Figure 6. In this mode the SS
pin is internally grounded, the on-timer is disabled, and bias
currents are reduced. Releasing the RON/SD pin allows
normal operation to resume. The voltage at the RON/SD pin
is between 1.5V and 3.0V, depending on V
IN and the RON
resistor.
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the recirculating current through the free-wheeling diode
(D1). Referring to Figure 3, when the buck switch is turned
off the inductor current flows through the load, into SGND,
through the sense resistor, out of ISEN and through D1. If
that current exceeds 1.25A the current limit comparator out-
put switches to delay the start of the next on-time period if
the voltage at FB is below 2.5V. The next on-time starts
when the current out of ISEN is below 1.25A and the voltage
at FB is below 2.5V. If the overload condition persists caus-
ing the inductor current to exceed 1.25A during each on-
time, that is detected at the beginning of each off-time. The
operating frequency is lower due to longer-than-normal off-
times.
Figure 7 illustrates the inductor current waveform. During
normal operation the load current is Io, the average of the
ripple waveform. When the load resistance decreases the
current ratchets up until the lower peak reaches 1.25A.
During the Current Limited portion of Figure 7, the current
ramps down to 1.25A during each off-time, initiating the next
on-time (assuming the voltage at FB is <2.5V). During each
on-time the current ramps up an amount equal to:
∆I=(V
IN -VOUT)xtON /L1
During this time the LM2695 is in a constant current mode,
with an average load current (I
OCL) equal to 1.25A +
∆I/2.
The current limit threshold can be increased by connecting
an external resistor between SGND and ISEN. The external
resistor will typically be less than 1
Ω. The peak current out of
SW and ISEN must not exceed 2A. The average current out
of SW must be less than 1.5A.
N - Channel Buck Switch and
Driver
The LM2695 integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The peak current
allowed through the buck switch is 2A, and the maximum
allowed average current is 1.5A. The gate driver circuit
works in conjunction with an external bootstrap capacitor
and an internal high voltage diode. A 0.022 µF capacitor (C4)
connected between BST and SW provides the voltage to the
driver during the on-time. During each off-time, the SW pin is
at approximately -1V, and C4 charges from V
CC through the
internal diode. The minimum off-time of 250 ns ensures a
minimum time each cycle to recharge the bootstrap capaci-
tor.
20170413
FIGURE 6. Shutdown Implementation
20170414
FIGURE 7. Inductor Current - Current Limit Operation
www.national.com
10


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