LTC3428_15 Datasheet, PDF (6/12 Pages) Linear Technology – 4A, 2MHz Dual Phase Step-Up DC/DC Converter in 3mm 3mm DFN

LTC3428
APPLICATIONS INFORMATION
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
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) = 1/5.9kΩ. 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 combination.
The internal 1.243V reference voltage is compared 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: 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.
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.
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
3428fb
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