LM27341_15 datasheet(10/44 Pages) TI

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RSENSE

PWM Logic

Driver

Switch

0.15:

Error Amplifier

SYNC

Freq. Foldback Amplifier

VREF

1.0V

1.13V

OVP Comparator

0.53V

Thermal

Shutdown

Current

Limit

LDO

PVIN

AVIN

BOOST

VOUT

GND

Under

Voltage

Lockout

Corrective

Ramp

ISENSE

Reset

Pulse

PWM

Comparator

Error

Signal

Oscillator

Internal

Compensation

Soft Start

Current Sense

Amplifier

LM27341, LM27342, LM27341-Q1, LM27342-Q1

SNVS497E – NOVEMBER 2008 – REVISED APRIL 2013

www.ti.com

Block Diagram

Figure 31.

APPLICATION INFORMATION

THEORY OF OPERATION

The LM27341/LM27342 is a constant-frequency, peak current-mode PWM buck regulator IC that delivers a 1.5

or 2A load current. The regulator has a preset switching frequency of 2 MHz. This high frequency allows the

LM27341/LM27342 to operate with small surface mount capacitors and inductors, resulting in a DC-DC converter

that requires a minimum amount of board space. The LM27341/LM27342 is internally compensated, which

reduces design time, and requires few external components.

The following operating description of the LM27341/LM27342 will refer to the Block Diagram (Figure 31) and to

the waveforms in Figure 32. The LM27341/LM27342 supplies a regulated output voltage by switching the internal

NMOS switch at a constant frequency and varying the 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 switch. During this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and the

inductor current (iL) increases with a linear slope. The current-sense amplifier measures iL, which generates an

output proportional to the switch current typically called the sense signal. 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 (VFB) and VREF. When the output of the PWM comparator goes high, the switch

turns off until the next switching cycle begins. During the switch off-time (tOFF), inductor current discharges

through the catch diode D1, which forces the SW pin (VSW) to swing below ground by the forward voltage (VD1)

of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage.

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