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LM25017 bảng dữ liệu(PDF) 10 Page - Texas Instruments

tên linh kiện LM25017
Giải thích chi tiết về linh kiện  LM25017 48-V, 650-mA Constant On-Time Synchronous Buck Regulator
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LM25017 bảng dữ liệu(HTML) 10 Page - Texas Instruments

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0.07 x VIN
TOFF(ILIM) =
VFB + 0.2V
2s
10
-10 x R
ON
TON =
VIN
LM25017
SNVS951D – DECEMBER 2012 – REVISED DECEMBER 2014
www.ti.com
Feature Description (continued)
At high-input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall
achievable output power. As an example, with the input at 48 V and switching at high frequency, the VCC
regulator may supply up to 7 mA of current resulting in 48 V × 7 mA = 336 mW of power dissipation. If the VCC
voltage is driven externally by an alternate voltage source between 8.55 V and 14 V, the internal regulator is
disabled. This reduces the power dissipation in the IC.
7.3.3 Regulation Comparator
The feedback voltage at FB is compared to an internal 1.225-V reference. In normal operation, when the output
voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225 V. The high side
switch will stay on for the on-time, causing the FB voltage to rise above 1.225 V. After the on-time period, the
high side switch will stay off until the FB voltage again falls below 1.225 V. During start up, the FB voltage will be
below 1.225 V at the end of each on-time, causing the high side switch to turn on immediately after the minimum
forced off-time of 144 ns. The high side switch can be turned off before the on-time is over if the peak current in
the inductor reaches the current limit threshold.
7.3.4 Overvoltage Comparator
The feedback voltage at FB is compared to an internal 1.62-V reference. If the voltage at FB rises above 1.62 V
the on-time pulse is immediately terminated. This condition can occur if the input voltage and/or the output load
changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225 V.
7.3.5 On-Time Generator
The on-time for the LM25017 device is determined by the RON resistor and is inversely proportional to the input
voltage (VIN), resulting in a nearly constant frequency as VIN is varied over the operating range. The on-time for
the LM25017 can be calculated using Equation 3.
(3)
See Figure 5. RON should be selected for a minimum on-time (at maximum VIN) greater than 100 ns for proper
operation. This requirement limits the maximum switching frequency for high VIN.
7.3.6 Current Limit
The LM25017 device contains an intelligent current limit off-timer. If the current in the buck switch exceeds 1.02
A, the present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of the off-
time is controlled by the FB voltage and the input voltage VIN. As an example, when FB = 0 V and VIN = 48 V, the
maximum off-time is set to 16
μs. This condition occurs when the output is shorted and during the initial part of
start-up. This VIN dependent off-time ensures safe short circuit operation up to the maximum input voltage of 48
V.
In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is
reduced. Reducing the off-time during less severe overloads reduces the amount of fold-back, recovery time, and
start-up time. The off-time is calculated as shown in Equation 4.
(4)
The current limit protection feature is peak limited. The maximum average output current will be less than the
peak.
7.3.7 N-Channel Buck Switch and Driver
The LM25017 device integrates an N-Channel Buck switch and associated floating high-voltage gate driver. The
gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high-voltage diode. A
0.01-uF ceramic capacitor connected between the BST pin and the SW pin provides the voltage to the driver
during the on-time. During each off-time, the SW pin is at approximately 0 V, and the bootstrap capacitor charges
from VCC through the internal diode. The minimum off-timer, set to 144 ns, ensures a minimum time each cycle to
recharge the bootstrap capacitor.
10
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