LTC3428

6

3428fb

DETAILED DESCRIPTION

The LTC3428 provides high efficiency, low noise power

for high current boost applications. A current mode

architecture with adaptive slope compensation provides

both simple loop compensation as well as excellent

pulse 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

simplify the external network to a simple R-C combination.

The internal 1.243V reference voltage is compared to the

voltage on FB to generate an error signal at the output of

ground programs the output voltage from 1.6V to 5.25V

using the equation:

Soft-Start: Where R1 and R2 are shown in Figure 3. 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 40ns.

The current signal leading edge is blanked for 40ns to

enhance noise rejection.

APPLICATIONS INFORMATION

Figure 1. Input Ripple Current Comparison

Between Single Phase and Two-Phase Boost

Converters with a 2A Load and 50% Duty Cycle

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 capacitor. 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 capacitors as

compared to a single-phase design.

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