LM5008EP Datasheet, PDF (8/16 Pages) Texas Instruments – High Voltage (100V) Step Down Switching Regulator

20187627
FIGURE 7. Efficiency vs Load Current vs V_IN
(Circuit of Figure 13)
down, V_CC under-voltage lockout, Gate drive under-voltage
lockout, Max Duty Cycle limit timer and the intelligent current
limit off timer.
Hysteretic Control Circuit Overview
The LM5008EP is a Buck DC-DC regulator that uses a control
scheme in which the on-time varies inversely with line voltage
(V_IN). Control is based on a comparator and the on-time one-
shot, with the output voltage feedback (FB) compared to an
internal reference (2.5V). If the FB level is below the reference
the buck switch is turned on for a fixed time 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 FB is still below the reference at
that time the switch will turn on again for another on-time pe-
riod. This will continue until regulation is achieved.
The LM5008EP operates in discontinuous conduction mode
at light load currents, and continuous conduction mode at
heavy load current. In discontinuous conduction mode, cur-
rent through the output inductor starts at zero and ramps up
to a peak during the on-time, then ramps back to zero before
the end of the off-time. The next on-time period starts when
the voltage at FB falls below the internal reference - until then
the inductor current remains zero. In this mode the operating
frequency is lower than in continuous conduction mode, and
varies with load current. Therefore at light loads the conver-
sion efficiency is maintained, since the switching losses re-
duce with the reduction in load and frequency. The discon-
tinuous operating frequency can be calculated as follows:
20187624
FIGURE 8. Output Voltage vs Load Current
(Circuit of Figure 13)
Functional Description
The LM5008EP Step Down Switching Regulator features all
the functions needed to implement a low cost, efficient, Buck
bias power converter. This high voltage regulator contains a
100 V N-Channel Buck Switch, is easy to implement and is
provided in the MSOP-8 and the thermally enhanced LLP-8
packages. The regulator is based on a hysteretic control
scheme using an on-time inversely proportional to V_IN. The
hysteretic control requires no loop compensation. Current
limit is implemented with forced off-time, which is inversely
proportional to V_OUT. This scheme ensures short circuit pro-
tection while providing minimum foldback. The Functional
Block Diagram of the LM5008EP is shown in Figure 1.
The LM5008EP can be applied in numerous applications to
efficiently regulate down higher voltages. This regulator is
well suited for 48 Volt Telecom and the new 42V Automotive
power bus ranges. Protection features include: Thermal Shut-
where R_L = the load resistance
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 discontinu-
ous mode frequency and remains relatively constant with load
and line variations. The approximate continuous mode oper-
ating frequency can be calculated as follows:
(1)
The output voltage (V_OUT) can be programmed by two exter-
nal resistors as shown in Figure 1. The regulation point can
be calculated as follows:
V_OUT = 2.5 x (R1 + R2) / R2
All hysteretic regulators regulate the output voltage based on
ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor C2. A minimum of
25mV to 50mV of ripple voltage at the feedback pin (FB) is
required for the LM5008EP. In cases where the capacitor
ESR is too small, additional series resistance may be required
(R3 in Figure 1).
For applications where lower output voltage ripple is required
the output can be taken directly from a low ESR output ca-
pacitor, as shown in Figure 9. However, R3 slightly degrades
the load regulation.
7
201876 Version 2 Revision 4 Print Date/Time: 2011/08/30 23:05:33
www.national.com