LTC3428

6

3428f

DETAILED DESCRIPTION

The LTC3428 provides high efficiency, low noise power

for high current boost applications. A current mode archi-

tecture with adaptive slope compensation provides both

simple loop compensation as well as excellent transient

width modulation control at high efficiency.

Oscillator: The per phase switching frequency is internally

set to a nominal value of 1MHz.

Current Sensing: Lossless current sensing converts the

peak current signal to a voltage which is summed with the

internal slope compensation. This summed signal is then

compared with the error amplifier output to provide a peak

current command for the PWM. Slope compensation is

internal to the IC and adapts to changes to the input

voltage, allowing the converter to provide the necessary

degree of slope compensation without causing a loss in

phase margin in the loop characteristic.

Error Amplifier: The error amplifier is a transconductance

often simplify the external network to a simple R-C com-

bination. The internal 1.243V reference voltage is com-

pared to the voltage on FB to generate an error signal at the

5.25V using the equation:

Soft-Start: An internal soft-start of approximately 1.5ms

is provided. This is a ramp signal that limits the peak

current until the internal soft-start voltage is greater than

the internal current limit voltage. The internal soft-start

capacitor is automatically discharged when the part is in

shutdown mode.

Current Limit: The current limit comparator in each phase

will shut off the N-channel MOSFET switches once the

current exceeds the current limit threshold, nominally

2.5A. The current limit delay to output is typically 50ns.

The current signal leading edge is blanked for 50ns to

enhance noise rejection.

Anti-Ringing Control: The antiringing control places an

impedance across the inductor of each phase to damp the

high frequency ringing on the SWA, SWB pins during

discontinuous mode operation. The LC ringing on the

switch pin due to the inductor and switch pin capacitance

is low energy, but can cause EMI radiation.

2-Phase Operation

The LTC3428 uses a two-phase architecture, rather than

the conventional single phase architecture used in most

other boost converters. The two phases are spaced 180°

apart. Two phase operation doubles the output ripple

frequency and provides a significant reduction in output

ripple current, minimizing the stress on the output capaci-

tor. Inductor (input) peak and ripple currents are also

reduced, allowing for the use of smaller, lower cost

inductors. The greatly reduced output ripple current also

minimizes the output capacitance requirement. The higher

frequency output ripple is easier to filter for lower noise

applications.

Input and output current comparisons for single and

2-phase converters are illustrated in Figures 1 and 2.

For the example illustrated in Figure 2, peak-to-peak

output ripple current was reduced by 85%, from 4.34A, to

0.64A, and peak inductor current was reduced by 53%,

from 4.34A to 2.02A. These reductions enable the use of

low profile, smaller valued inductors and output capaci-

tors as compared to a single-phase design.

APPLICATIO S I FOR ATIO

Figure 1. Input Ripple Current Comparison

Between Single Phase and Two-Phase Boost

Converters with a 2A Load and 50% Duty Cycle

TIME (µs)

0

4.4

4.3

4.2

4.1

4.0

3.9

3.8

3.7

3.6

1.6

3428 F01

0.4

0.2

0.6

1.0

1.4

1.8

0.8

1.2

2.0

1 PHASE

CONVERTER

2 PHASE

CONVERTER