Datasheet

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
= t
ON
x F
S
=
F
S
=
V
OUT
x (V
IN
± 1.4V)
1.18 x 10
-10
x (R
ON
+ 1.4 k:) x V
IN
LM5010A
LM5010A-Q1
SNVS376E –OCTOBER 2005–REVISED FEBRUARY 2013 www.ti.com
Control Circuit Overview
The LM5010A 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 a fixed 260 ns off-time, or until the FB voltage falls below the
reference, whichever is longer. The buck switch then turns on for another on-time period. Referring to the Block
Diagram, the output voltage is set by R1 and R2. The regulated output voltage is calculated as follows:
V
OUT
= 2.5V x (R1 + R2) / R2 (1)
The LM5010A requires a minimum of 25 mV of ripple voltage at the FB pin for stable fixed-frequency operation. If
the output capacitor’s ESR is insufficient additional series resistance may be required (R3 in the Block Diagram).
The LM5010A 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
decaying to 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 operating frequency in the continuous conduction mode is calculated as follows:
(2)
The buck switch duty cycle is equal to:
(3)
Under light load conditions, the LM5010A operates in discontinuous conduction mode, with zero current flowing
through the inductor for a portion of the off-time. The operating frequency is always lower than that of the
continuous conduction mode, and the switching frequency varies with load current. Conversion efficiency is
maintained at a relatively high level at light loads since the switching losses diminish as the power delivered to
the load is reduced. The discontinuous mode operating frequency is approximately:
(4)
where R
L
= the load resistance.
Start-Up Bias Regulator (V
CC
)
A high voltage bias regulator is integrated within the LM5010A. The input pin (VIN) can be connected directly to
line voltages between 6V and 75V. Referring to the block diagram and the graph of V
CC
vs. V
IN
, when V
IN
is
between 6V and the bypass threshold (nominally 8.9V), the bypass switch (Q2) is on, and V
CC
tracks V
IN
within
100 mV to 150 mV. The bypass switch on-resistance is approximately 50Ω, with inherent current limiting at
approximately 100 mA. When VIN is above the bypass threshold, Q2 is turned off, and V
CC
is regulated at 7V.
The V
CC
regulator output current is limited at approximately 15 mA. When the LM5010A is shutdown using the
RON/SD pin, the V
CC
bypass switch is shut off, regardless of the voltage at VIN.
When V
IN
exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 - 3 µs. The
capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its
absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to
the LM5010A pins.
In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An
auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin (D2 in Figure 8) to shut off the
VCC regulator, reducing internal power dissipation. The current required into the VCC pin is shown in the Typical
Performance Characteristics. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less
than V
IN
.
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