LTC3428 Datasheet, PDF (6/12 Pages) Linear Technology – 4A, 2MHz Dual Phase Step-Up DC/DC Converter in 3mm 3mm DFN

LTC3428
APPLICATIO S I FOR ATIO
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
response. The low R_DS(ON) switches provide the 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
amplifier with a transconductance (g_m) = 1/7.5kΩ. A
simple compensation network is placed from V_C to ground.
The internal 5pF capacitor between V_C and ground will
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
output of the error amplifier (V_C). A voltage divider from
V_OUT to ground programs the output voltage from 1.6V to
5.25V using the equation:
V_OUT = 1.243V • ( 1+ R1/R2)
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.
6
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.
4.4
1 PHASE
4.3
CONVERTER
4.2
2 PHASE
4.1
CONVERTER
4.0
3.9
3.8
3.7
3.6
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
TIME (µs)
3428 F01
Figure 1. Input Ripple Current Comparison
Between Single Phase and Two-Phase Boost
Converters with a 2A Load and 50% Duty Cycle
3428f