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LM25017 数据表(PDF) 9 Page - Texas Instruments

部件名 LM25017
功能描述  LM25017 48-V, 650-mA Constant On-Time Synchronous Buck Regulator
Download  31 Pages
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制造商  TI [Texas Instruments]
网页  http://www.ti.com
标志 TI - Texas Instruments

LM25017 数据表(HTML) 9 Page - Texas Instruments

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FB
SW
L1
COUT
RFB2
VOUT
RC
LM25017
+
RFB1
VOUT
(low ripple)
RFB2 + RFB1
VOUT = 1.225V x
RFB1
VOUT
CSW =
K x RON
LM25017
www.ti.com
SNVS951D – DECEMBER 2012 – REVISED DECEMBER 2014
7.3 Feature Description
7.3.1 Control Overview
The LM25017 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the
reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input
voltage and the programming resistor (RON). Following the on-time the switch remains off until the FB voltage
falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer.
When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck
switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB
voltage is approximately equal to 1.225 V (typ).
In a synchronous buck converter, the low side (sync) FET is ‘on’ when the high side (buck) FET is ‘off’. The
inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’.
There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative
direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of
the output loading. The operating frequency remains relatively constant with load and line variations. The
operating frequency can be calculated as shown in Equation 1.
where
K = 9 x 10
–11
(1)
The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated
as shown in Equation 2.
(2)
This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor (COUT). A minimum of 25-mV ripple voltage at the feedback pin (FB) is
required for the LM25017 device. In cases where the capacitor ESR is too small, additional series resistance
may be required (RC in Figure 10).
For applications where lower output voltage ripple is required, the output can be taken directly from a low ESR
output capacitor, as shown in Figure 10. However, RC slightly degrades the load regulation.
Figure 10. Low Ripple Output Configuration
7.3.2 VCC Regulator
The LM25017 device contains an internal high-voltage linear regulator with a nominal output of 7.6 V. The input
pin (VIN) can be connected directly to the line voltages up to 48 V. The VCC regulator is internally current limited
to 30 mA. The regulator sources current into the external capacitor at VCC. This regulator supplies current to
internal circuit blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the
VCC pin reaches the undervoltage lockout (VCC UVLO) threshold of 4.5 V, the IC is enabled.
An internal diode connected from VCC to the BST pin replenishes the charge in the gate drive bootstrap
capacitor when SW pin is low.
Copyright © 2012–2014, Texas Instruments Incorporated
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