LM2734Z_15 Datasheet, PDF (6/32 Pages) Texas Instruments – LM2734Z/LM2734ZQ Thin SOT 1A Load Step-Down DC-DC Regulator

LM2734Z
SNVS334E – JANUARY 2005 – REVISED APRIL 2013
Block Diagram
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ON ^EN
OFF
V_IN
Internal
Regulator
and
Enable
Circuit
Under
Voltage
Lockout
Oscillator _Reset
Pulse
I_SENSE
+
+
Corrective Ramp
Current-Sense Amplifier
+
-
Thermal
Shutdown
R_SENSE
BOOST
Current
Limit
Output
Control
Logic
Driver
OVP
Comparator
0.3:
Switch _SW
-
+
PWM
Comparator
Error
Signal
Internal
-
Compensation
+
Error Amplifier
0.88V
+
-
FB
+ ^VREF
- _0.8V
D2
V_BOOST
C_BOOST
V_SW ^L
D ^IL
1
R
1
R
2
GND
V_IN
C_IN
V_OUT
C_OUT
APPLICATION INFORMATION
THEORY OF OPERATION
The LM2734Z is a constant frequency PWM buck regulator IC that delivers a 1A load current. The regulator has
a preset switching frequency of 3MHz. This high frequency allows the LM2734Z to operate with small surface
mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space.
The LM2734Z is internally compensated, so it is simple to use, and requires few external components. The
LM2734Z uses current-mode control to regulate the output voltage.
The following operating description of the LM2734Z will refer to the Simplified Block Diagram (Block Diagram)
and to the waveforms in Figure 9. The LM2734Z 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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