Datasheet

FB
SW
L1
C2
R3
LM5010
R1
R2
V
OUT2
F
S
=
V
OUT
2
x L1 x 1.4 x 10
20
R
L
x (R
ON
)
2
DC =
t
ON
t
ON
+ t
OFF
V
OUT
V
IN
=
F
S
=
V
OUT
1.18 x 10
-10
x R
ON
LM5010
SNVS307F –SEPTEMBER 2004–REVISED FEBRUARY 2013 www.ti.com
Hysteretic Control Circuit Overview
The LM5010 buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-timer,
with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB voltage is below the
reference the buck switch is turned on for a time period determined by the input voltage and a programming
resistor (R
ON
). Following the on-time the switch remains off for 265 ns, or until the FB voltage falls below the
reference, whichever is longer. The buck switch then turns on for another on-time period. Typically when the load
current increases suddenly, the off-times are temporarily at the minimum of 265 ns. Once regulation is
established, the off-time resumes its normal value. The output voltage is set by two external resistors (R1, R2).
The regulated output voltage is calculated as follows:
V
OUT
= 2.5V x (R1 + R2) / R2 (1)
Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR
for the output capacitor C2. The LM5010 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases
where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Typical Application
Circuit and Block Diagram).
When in regulation, the LM5010 operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode current always flows
through the inductor, never reaching zero during the off-time. In this mode 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. The approximate operating frequency is calculated as follows:
(2)
The buck switch duty cycle is approximately equal to:
(3)
At low load current, the circuit operates in discontinuous conduction mode, during which the inductor current
ramps up from zero 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 reference - until then the inductor current
remains zero, and the load current is supplied by the output capacitor (C2). In this mode 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 reduce with the reduction in load and frequency. The approximate
discontinuous operating frequency can be calculated as follows:
(4)
where R
L
= the load resistance.
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.
Figure 9. Low Ripple Output Configuration
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