Datasheet

F =
V
OUT
1.25 x 10
-10
x R
ON
F =
V
OUT
2
x L x 1.28 x 10
20
R
L
x (R
ON
)
2
LM5008
www.ti.com SNVS280G –APRIL 2004–REVISED MARCH 2013
FUNCTIONAL DESCRIPTION
The LM5008 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 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 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 protection while providing minimum foldback. The Functional Block
Diagram of the LM5008 is shown in Figure 2.
The LM5008 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 Shutdown, 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 LM5008 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 period. This will continue until regulation is achieved.
The LM5008 operates in discontinuous conduction mode at light load currents, and continuous conduction mode
at heavy load current. In discontinuous conduction mode, current 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 conversion efficiency is maintained, since the switching losses reduce with
the reduction in load and frequency. The discontinuous operating frequency can be calculated as follows:
(1)
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 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 (V
OUT
) 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)
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 LM5008. In cases where the capacitor ESR is too small, additional series
resistance may be required (R3 in Figure 2).
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, R3 slightly degrades the load regulation.
Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 7
Product Folder Links: LM5008