Datasheet

cv
1.6 MHz
S
R
R
Q
+
-
+
-
+
-


-
+
EN
V
IN
Thermal
SHDN
SW
I
LIMIT
NMOS
UVLO = 2.3V
I
SENSE-AMP
Internal
Compensation
Soft-Start
Corrective - Ramp
Oscillator
Control Logic
V
REF
= 1.255V
FB
GND
LM2735
SNVS485F –JUNE 2007–REVISED APRIL 2013 www.ti.com
Simplified Internal Block Diagram
Figure 18. Simplified Block Diagram
APPLICATION INFORMATION
THEORY OF OPERATION
The LM2735 is a constant frequency PWM boost regulator IC that delivers a minimum of 2.1A peak switch
current. The regulator has a preset switching frequency of either 520 kHz or 1.60 MHz. This high frequency
allows the LM2735 to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter
that requires a minimum amount of board space. The LM2735 is internally compensated, so it is simple to use,
and requires few external components. The LM2735 uses current-mode control to regulate the output voltage.
The following operating description of the LM2735 will refer to the Simplified Internal Block Diagram (Figure 18)
the simplified schematic (Figure 19), and its associated waveforms (Figure 20). The LM2735 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
) decreases to approximately GND, 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
sensed 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 diode D1, which forces the SW pin to swing to the output voltage plus the forward voltage
(V
D
) of the diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage .
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