Datasheet
UVLO
VIN
VCC
SW Pin
Inductor
Current
SS Pin
t2
V
OUT
7.0V
2.5V
t1
LM2694
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SNVS444A –MAY 2006–REVISED APRIL 2013
Figure 9. Startup Sequence
Functional Description
The LM2694 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient
buck bias power converter capable of supplying at least 0.6A to the load. This high voltage regulator contains a
30V N-Channel buck switch, is easy to implement, and is available in the TSSOP-14 and the thermally enhanced
WSON-10 packages. The regulator’s operation is based on a constant on-time control scheme, where the on-
time is determined by V
IN
. This feature allows the operating frequency to remain relatively constant with load and
input voltage variations. The feedback control requires no loop compensation resulting in very fast load transient
response. The valley current limit detection circuit, internally set at 0.62A, holds the buck switch off until the high
current level subsides. This scheme protects against excessively high currents if the output is short-circuited
when V
IN
is high. The functional block diagram is shown in Typical Application Circuit and Block Diagram.
The LM2694 can be applied in numerous applications to efficiently regulate down higher voltages. Additional
features include: Thermal shutdown, V
CC
under-voltage lockout, gate drive under-voltage lockout, and maximum
duty cycle limiter.
Control Circuit Overview
The LM2694 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 a minimum of 265 ns, and until the FB voltage
falls below the reference. The buck switch then turns on for another on-time period. Typically, during start-up, or
when the load current increases suddenly, the off-times are at the minimum of 265 ns. Once regulation is
established, the off-times are longer.
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