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LM25085MYX 데이터시트(PDF) 10 Page - Texas Instruments

부품명 LM25085MYX
상세설명  LM25085, LM25085-Q1 42V Constant On-Time PFET Buck Switching Controller
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LM25085MYX 데이터시트(HTML) 10 Page - Texas Instruments

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FS =
VOUT x (VIN - 1.56V + RT/3167)
VIN x [(1.45 x 10
-7
x (RT + 1.4)) + (tD x (VIN - 1.56V + RT/3167))]
tON =
(VIN - 1.56V + RT/3167)
1.45 x 10
-7
x (RT + 1.4)
+ 50 ns
LM25085, LM25085-Q1
SNVS593H – OCTOBER 2008 – REVISED MARCH 2013
www.ti.com
The LM25085 is available in both an 8 pin HVSSOP-PowerPAD package and an 8 pin WSON package with an
exposed pad to aid in heat dissipation. An 8 pin VSSOP package without an exposed pad is also available.
REGULATION CONTROL CIRCUIT
The LM25085 buck DC-DC controller employs a control scheme based on a comparator and a one-shot on-
timer, with the output voltage feedback compared to an internal reference voltage (1.25V). When the FB pin
voltage falls below the feedback reference, Q1 is switched on for a time period determined by the input voltage
and a programming resistor (RT). Following the on-time Q1 remains off until the FB voltage falls below the
reference. Q1 is then switched on for another on-time period. The output voltage is set by the feedback resistors
(RFB1, RFB2 in Block Diagram). The regulated output voltage is calculated as follows:
VOUT = 1.25V x (RFB2+ RFB1)/ RFB1
(2)
The feedback voltage supplied to the FB pin is applied to a comparator rather than a linear amplifier. For proper
operation sufficient ripple amplitude is necessary at the FB pin to switch the comparator at regular intervals with
minimum delay and noise susceptibility. This ripple is normally obtained from the output voltage ripple attenuated
through the feedback resistors. The output voltage ripple is a result of the inductor’s ripple current passing
through the output capacitor’s ESR, or through a resistor in series with the output capacitor. Multiple methods are
available to ensure sufficient ripple is supplied to the FB pin, and three different configurations are discussed in
Applications Information.
When in regulation, the LM25085 operates in continuous conduction mode at medium to heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode the inductor’s current is
always greater than zero, and the operating frequency remains relatively constant with load and line variations.
The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. In
discontinuous conduction mode, where the inductor’s current reaches zero during the off-time, the operating
frequency is lower than in continuous conduction mode and varies with load current. Conversion efficiency is
maintained at light loads since the switching losses are reduced with the reduction in load and frequency.
If the voltage at the FB pin exceeds 1.6V due to a transient overshoot or excessive ripple at VOUT the internal
over-voltage comparator immediately switches off Q1. The next on-time period starts when the voltage at FB falls
below the feedback reference voltage.
ON-TIME TIMER
The on-time of the PFET gate drive output (PGATE pin) is determined by the resistor (RT) and the input voltage
(VIN), and is calculated from:
where
RT is in kΩ
(3)
The minimum on-time, which occurs at maximum VIN, should not be set less than 150ns (see CURRENT
LIMITING). The buck regulator effective on-time, measured at the SW node (junction of Q1, L1, and D1) is
typically longer than that calculated in Equation 3 due to the asymmetric delay of the PFET. The on-time
difference caused by the PFET switching delay can be estimated as the difference of the turn-off and turn-on
delays listed in the PFET data sheet. Measuring the difference between the on-time at the PGATE pin versus the
SW node in the actual application circuit is also recommended.
In continuous conduction mode, the inverse relationship of tON with VIN results in a nearly constant switching
frequency as VIN is varied. The operating frequency can be calculated from:
where
RT is in kΩ
tD is equal to 50ns plus the PFET’s delay difference
(4)
10
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Product Folder Links: LM25085 LM25085-Q1


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