Datasheet

F =
V
OUT
1.42 x 10
-10
x R
ON
F =
V
OUT
2
x L
1 x 10
-20
x R
Load
x (R
ON
)
2
LM5007
www.ti.com SNVS252F –SEPTEMBER 2003–REVISED MARCH 2013
DETAILED OPERATING DESCRIPTION
The LM5007 Step Down Switching Regulator features all of the functions needed to implement low cost, efficient,
Buck bias regulators. This high voltage regulator contains an 80V, 0.7A N-Channel Buck Switch. The device is
easy to apply and is provided in the VSSOP-8 and the thermally enhanced WSON-8 packages. The regulator is
based on a hysteretic control scheme using an on time inversely proportional to Vin. This feature allows the
operating frequency to remain relatively constant with load and input voltage variations. The hysteretic control
requires no control loop compensation, while providing very fast load transient response. An intelligent current
limit scheme is implemented in the LM5007 with forced off time, after current limit detection, which is inversely
proportional to Vout. This current limiting scheme reduces load current foldback. Additional protection features
include: Thermal Shutdown, Vcc undervoltage lockout, Gate drive undervoltage lockout and Max Duty Cycle
limiter. The LM5007 can be applied in numerous applications to efficiently regulate step down higher voltage
inputs. This regulator is well suited for 48 Volt Telcom and the new 42V Automotive power bus ranges.
Hysteretic Control Circuit Overview
The LM5007 is a Buck DC-DC regulator that uses an on time control scheme. The on time is programmed by an
external resistor and varies inversely with line input voltage (Vin). The core regulation elements of the LM5007
are the feedback comparator and the on time one-shot. The regulator output voltage is sensed at the feedback
pin (FB) and is compared to an internal reference voltage (2.5V). If the FB signal is below the reference voltage,
the buck switch is turned on for a fixed time pulse determined by the line voltage and a programming resistor
(R
ON
). Following the on period the switch will remain off for at least the minimum off timer period of 300ns. If the
FB pin voltage is still below the reference after the 300ns off time, the switch will turn on again for another on
time period. This switching behavior will continue until the FB pin voltage reaches the reference voltage level.
The LM5007 operates in discontinuous conduction mode at light load currents or continuous conduction mode at
heavier load currents. In discontinuous conduction mode, current through the output inductor starts at zero and
ramps up to a peak value during the buck switch on time and then back to zero during the off time. The inductor
current remains at zero until the next on time period starts when FB falls below the internal reference. In
discontinuous mode the operating frequency can be relatively low and will vary with load. Therefore at light loads
the conversion efficiency is maintained, since the switching losses decrease with the reduction in load current
and switching frequency. The approximate discontinuous mode operating frequency can be calculated as follows:
(1)
In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero.
In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively
constant with load and line variations. The approximate continuous mode operating frequency can be calculated
as follows:
(2)
The output voltage (Vout) can be programmed by two external resistors as shown in Figure 2. The regulation
point can be calculated as follows:
V
OUT
= 2.5 x (R1 + R2) / R2 (3)
The feedback comparator in hysteretic regulators depend upon the output ripple voltage to switch the output
transistor on and off at regular intervals. In order for the internal comparator to respond quickly to changes in
output voltage, proportional to inductor current, a minimum amount of capacitor Equivalent Series Resistance
(ESR) is required. A ripple voltage of 25mV to 50mV is recommended at the feedback pin (FB) for stable
operation. In cases where the intrinsic capacitor ESR is too small, additional series resistance may be added.
For applications where lower output voltage ripple is required the load can be connected directly to the low ESR
output capacitor, as shown in Figure 2. The series resistor (R) will degrade the load regulation. Another
technique for enhancing the ripple voltage at the FB pin is to place a capacitor in parallel with the feedback
divider resistor R1. The addition of the capacitor reduces the attenuation of the ripple voltage from the feedback
divider
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