Datasheet
L
R1
R2
D
1
D2
BOOST
Output
Control
Logic
Current
Limit
Thermal
Shutdown
Under
Voltage
Lockout
Corrective Ramp
Reset
Pulse
PWM
Comparator
Current-Sense Amplifier
R
SENSE
+
+
Internal
Regulator
and
Enable
Circuit
Oscillator
Driver
0.3:
Switch
Internal
Compensation
SW
EN
FB
GND
Error Amplifier
-
+
V
REF
1.25V
C
OUT
ON
OFF
V
BOOST
V
SW
+
-
C
BOOST
V
OUT
C
IN
V
IN
V
IN
I
SENSE
+
-
+
-
+
-
1.375V
OVP
Comparator
Error
Signal
-
+
I
L
LM2736
www.ti.com
SNVS316F –SEPTEMBER 2004–REVISED APRIL 2013
Block Diagram
Figure 19.
APPLICATION INFORMATION
THEORY OF OPERATION
The LM2736 is a constant frequency PWM buck regulator IC that delivers a 750mA load current. The regulator
has a preset switching frequency of either 550kHz (LM2736Y) or 1.6MHz (LM2736X). These high frequencies
allow the LM2736 to operate with small surface mount capacitors and inductors, resulting in DC/DC converters
that require a minimum amount of board space. The LM2736 is internally compensated, so it is simple to use,
and requires few external components. The LM2736 uses current-mode control to regulate the output voltage.
The following operating description of the LM2736 will refer to the Simplified Block Diagram (Figure 19) and to
the waveforms in Figure 20. The LM2736 supplies a regulated output voltage by switching the internal NMOS
control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the
reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the
internal NMOS control switch. During this on-time, the SW pin voltage (V
SW
) swings up to approximately V
IN
, and
the inductor current (I
L
) increases with a linear slope. I
L
is measured by the current-sense amplifier, which
generates an output proportional to the switch current. The sense signal is summed with the regulator’s
corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the
feedback voltage and V
REF
. When the PWM comparator output goes high, the output switch turns off until the
next switching cycle begins. During the switch off-time, inductor current discharges through Schottky diode D1,
which forces the SW pin to swing below ground by the forward voltage (V
D
) of the catch diode. The regulator
loop adjusts the duty cycle (D) to maintain a constant output voltage.
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