LM5010 EVAL/NOPB National Semiconductor, LM5010 EVAL/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM5010 EVAL/NOPB
• Manufacturer: National Semiconductor

Specifications of LM5010 EVAL/NOPB

• Lead Free Status / Rohs Status: Compliant

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Hysteretic Control Circuit
Overview
Typically when the load current increases suddenly, the off-
times are temporarily at the minimum of 265 ns. Once regu-
lation 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:
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 Figure 1 ).
When in regulation, the LM5010 operates in continuous
conduction mode at heavy load currents and discontinuous
conduction mode at light load currents. In continuous con-
duction 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 calcu-
lated as follows:
The buck switch duty cycle is approximately equal to:
Start-up Regulator (V
The startup regulator is integral to the LM5010. The input pin
(V
The V
limited to 10 mA. Upon power up the regulator sources
current into the external capacitor at V
capacitor at V
V
(UVLO) of 5.8V (t1 in Figure 8), at which time the buck switch
is enabled, and the softstart pin is released to allow the
softstart capacitor (C6) to charge up. V
its regulated value as the softstart voltage increases (t2 in
Figure 8).
CC
IN
) can be connected directly to line voltages up to 75V.
voltage to reach the under-voltage lockout threshold
CC
output is regulated at 7.0V,
CC
V
OUT
, approximately 58 µs are required for the
(Continued)
= 2.5V x (R1 + R2) / R2
CC
CC
OUT
±
FIGURE 9. Low Ripple Output Configuration
)
6%, and is current
(C3). With a 0.1 µF
then increases to
(1)
(2)
10
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 refer-
ence - 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 cal-
culated as follows:
where R
For applications where lower output voltage ripple is re-
quired the output can be taken directly from a low ESR
output capacitor as shown in Figure 9. However, R3 slightly
degrades the load regulation.
The minimum input operating voltage is determined by the
regulator’s dropout voltage, the V
()5.65V), and the frequency. When V
falling threshold the V
switch and ground the softstart pin. If V
loaded, the minimum input voltage increases since the out-
put impedance at V
and 3. In applications involving a high value for V
power dissipation in the startup regulator is a concern, an
auxiliary voltage can be diode connected to the V
(Figure 10). Setting the auxiliary voltage to between 7.5V
and 14V shuts off the internal regulator, reducing internal
power dissipation. The current required into the V
shown in Figure 4. Internally a diode connects V
L
= the load resistance.
CC
CC
is )140Ω at low V
UVLO activates to shut off the buck
20119915
CC
UVLO falling threshold
CC
IN
CC
. See Figures 2
falls below the
is externally
CC
CC
IN
to V
CC
where
pin is
IN
pin
(3)
(4)
.