LTC3425 Datasheet, PDF (14/24 Pages) Linear Technology – 5A, 8MHz, 4-Phase Synchronous Step-Up DC/DC Converter

LTC3425
U
OPERATIO
98
T_A = 25°C
96 ^VIN ^{= 2.4V}
V_OUT = 3.3V
94 1MHz/PHASE
92
90
4 PHASE
88
86
2 PHASE 3 PHASE
84
82
80
100
1000
LOAD (mA)
10000
3425 G13
Figure 4. LTC3425 Efficiency vs
Load for 2-, 3- and 4-Phase Operation
Low Noise Fixed Frequency Operation
Shutdown: The part is shut down by pulling SHDN below
0.25V and made active by pulling the pin above 1V. Note
that SHDN can be driven above V_IN or V_OUT, as long as it
is limited to less than 5.5V.
Soft-Start: The soft-start time is programmed with an
external capacitor to ground on SS. An internal current
source charges it with a nominal 2.5µA (1µA while in start-
up mode when V_IN and V_OUT are both below 2.3V). The
voltage on the soft-start pin (in conjunction with the
external resistor on the I_LIM pin) is used to control the peak
current limit until the voltage on the capacitor exceeds
1.6V, at which point the external resistor sets the peak
current. In the event of a commanded shutdown or a
thermal shutdown, the capacitor is discharged automati-
cally. Note that Burst Mode operation is inhibited during
the soft-start time.
t(ms) = C_SS(µF) • 320
Oscillator: The frequency of operation is set through a re-
sistor from the R_T pin to ground. An internally trimmed
timing capacitor resides inside the IC. The internal
oscillator frequency is then divided by four to generate the
four phases, each phase shifted by 90°. The oscillator fre-
quency and resulting switching frequency of each of the four
phases are calculated using the following formula:
f_OSC
=
60
R_T
f_SWITCH
=
f_OSC
4
=
15
R_T
where f_OSC is in MHz and R_T is in kΩ.
The oscillator can be synchronized with an external clock
applied to SYNCIN. When synchronizing the oscillator, the
free running frequency must be set to an approximately
30% lower frequency than the desired synchronized fre-
quency. SYNCOUT is provided for synchronizing two or
more devices. The output sync pulse is 180° out of phase
from the internal oscillator, allowing two devices to be
synchronized to create an 8-phase converter. Note that in
Burst Mode operation, the oscillator is turned off and
SYNCOUT is driven low.
In fixed frequency operation, the minimum on-time before
pulse skipping occurs (at light load) is typically 110ns.
Current Sensing: Lossless current sensing converts the
peak current signal to a voltage to sum in with the internal
slope compensation. This summed signal is compared to
the error amplifier output to provide a peak current control
command for the PWM. The internal slope-compensation
is adaptive to the input and output voltage, therefore the
converter provides the proper amount of slope compensa-
tion to ensure stability, but not an excess to cause a loss
of phase margin in the converter.
Error Amp: The error amplifier is a transconductance
amplifier with its positive input internally connected to the
1.22V reference and its negative input connected to FB. A
simple compensation network is placed from COMP to
ground. Internal clamps limit the minimum and maximum
error amp output voltage for improved large-signal tran-
sient response. During Burst Mode operation, the com-
pensation pin is high impedance, however clamps limit the
voltage on the external compensation network, preventing
the compensation capacitor from discharging to zero.
3425f
14