LTC1436 Linear Technology Corporation, LTC1436 Datasheet

LTC1436

Manufacturer Part Number

LTC1436

Description

Manufacturer

Linear Technology Corporation

Datasheet

1.LTC1436.pdf (2 pages)

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Electromagnetic interference (EMI) is a potential problem

for the circuit designer. Switching regulators can cause

EMI in many products. Linear Technology has developed

new techniques like spread-spectrum modulation, phase-

locked synchronization and Adaptive Power

can reduce the amount of unwanted interference.

New IC Solves Old Problems

The LTC

synchronous step-down switching regulator that con-

trols external N-channel MOSFETs for very efficient power

conversion. It also features Adaptive Power mode, which

provides constant frequency switching with good effi-

ciency at light load currents. Figure 1a shows the audio

frequencies generated by the 5V output of the circuit in

Figure 2, while supplying 3mA of load current (0.1% of full

load) using a cycle-skipping mode of operation. This

mode may cause many cycles to be skipped between

bursts of energy to the output capacitor. These energy

bursts intrude into the audio band at sufficiently low

output currents. Figure 1b shows that audio frequency

noise is completely eliminated by the Adaptive Power

mode under the same conditions.

Traditionally, efficiency is sacrificed to accomplish the

audio frequency response shown in Figure 1b. Large

synchronous MOSFETs are used to force continuous in-

ductor current at the switching frequency, regardless of the

load. The associated gate charge losses and losses caused

1196/141

LTC1436-PLL Low Noise Switching Regulator Helps Control EMI

Design Note 141

John Seago

Figure 1a. Audio Frequency Generation in

Cycle-Skipping Mode

1436-PLL is a constant-frequency, current mode,

–120dBm

–100dBm

– 40dBm

– 60dBm

– 80dBm

–20dBm

10Hz

10V

BW = 100Hz

OUT

AT 3mA

10kHz

DN141 F01A

20kHz

mode that

by relatively large inductor ripple current result in very poor

efficiency at light loads. The Adaptive Power mode uses

only the small (SOT-23) MOSFET, Q3 and D2 in a conven-

tional buck mode to allow constant frequency, discontinu-

ous inductor current operation, which greatly decreases

power loss. The gate-to-source capacitance of the small

MOSFET is significantly less than either of the two large

MOSFETs. Depending on component selection, there can

be a 50-to-1 difference in gate-to-source capacitance be-

tween Q3 and Q1/Q2, so that Q3 requires only 2% of the

gate drive power (loss) of Q1/Q2. This provides a substan-

tial increase in efficiency at light loads.

New Feature Provides New EMI Control

In addition to audio frequency suppression, the LTC1436-

PLL has three additional RF EMI control mechanisms:

1) The LTC1436-PLL allows switching frequency modula-

tion to spread the spectrum of switching noise. Through

frequency modulation, peak energy is decreased and spread

over a wide range of frequencies as shown in Figure 3. The

normal 190kHz switching frequency and its harmonics are

shown by the black trace. The colored trace shows the

result of modulating the phase-locked loop lowpass filter

(PLL LPF) pin with a 100Hz sawtooth waveform. Switching

frequency energy is reduced by over 20dB when modu-

lated; second and third harmonics are attenuated even

more in this example. Figure 4 shows the spectrum out to

Adaptive Power is a trademark of Linear Technology Corporation.

, LTC and LT are registered trademarks of Linear Technology Corporation.

Figure 1b. Audio Frequency Amplitude with Adaptive

Power Mode Operation

–100dBm

–120dBm

–20dBm

–40dBm

–60dBm

–80dBm

10Hz

10V

BW = 100Hz

OUT

AT 3mA

10kHz

DN141 F01B

20kHz

Related parts for LTC1436

LTC1436 Summary of contents

Page 1

... LTC1436-PLL Low Noise Switching Regulator Helps Control EMI Design Note 141 John Seago Electromagnetic interference (EMI potential problem for the circuit designer. Switching regulators can cause EMI in many products. Linear Technology has developed new techniques like spread-spectrum modulation, phase- locked synchronization and Adaptive Power can reduce the amount of unwanted interference ...

Page 2

... PNP transistor for an additional low noise, linear regulated output. The LTC1437 has all the features of the LTC1436- PLL plus an internal comparator with reference that can be used to detect a low-battery condition or provide other useful functions ...

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