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
LM5009
www.ti.com SNVS402G –FEBRUARY 2006–REVISED FEBRUARY 2013
FUNCTIONAL DESCRIPTION
The LM5009 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 control scheme using an on-time inversely proportional to V
IN
. The control scheme 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
LM5009 is shown in Figure 8.
The LM5009 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator
is well suited for 48 Volt Telecom and the 42V Automotive power bus ranges. Additional 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.
Control Circuit Overview
The LM5009 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 300 ns. 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, at
which time the off-time increases based on the required duty cycle.
The LM5009 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
) is programmed by two external resistors as shown in Figure 8. The regulation point is
calculated as follows:
V
OUT
= 2.5 x (R1 + R2) / R2 (3)
This regulator regulates 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 of ripple voltage at the feedback pin (FB) is
required for the LM5009. In cases where the capacitor ESR is too small, additional series resistance may be
required (R3 in Figure 8).
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 9. However, R3 slightly degrades the load regulation.
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