LTC3443
7
3443fa
OPERATIO
U
The LTC3443 provides high efficiency, low noise power for
applications such as portable instrumentation. The LTC
proprietary topology allows input voltages above, below or
equal to the output voltage by properly phasing the output
switches. The error amp output voltage on the V
C
pin de-
termines the output duty cycle of the switches. Since the
V
C
pin is a filtered signal, it provides rejection of frequen-
cies from well below the switching frequency. The low
R
DS(ON)
, low gate charge synchronous switches provide
high frequency pulse width modulation control at high
efficiency. Schottky diodes across the synchronous switch
D and synchronous switch B are not required, but provide
a lower drop during the break-before-make time (typically
15ns). The addition of the Schottky diodes will improve peak
efficiency by typically 1% to 2%. High efficiency is achieved
at light loads when Burst Mode operation is entered and
when the IC’s quiescent current is a low 28μA.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is factory trimmed to 600kHz.
The oscillator can be synchronized with an external clock
applied to the MODE/SYNC pin. A clock frequency of twice
the desired switching frequency and with a pulse width
of at least 100ns is applied. The oscillator sync range
is 690kHz to 1.2MHz (1.38MHz to 2.4MHz sync
frequency).
Error Amp
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier to obtain stability of the converter. The SHDN/SS
pin will clamp the error amp output, V
C
, to provide a soft-
start function.
Supply Current Limit
The current limit amplifier will shut PMOS switch A off
once the current exceeds 4A typical. Before the switch
current limit, the average current limit amp (3.2A typical)
will source current into the FB pin to drop the output
voltage. The current amplifier delay to output is typically
50ns.
Reverse Current Limit
The reverse current limit amplifier monitors the inductor
current from the output through switch D. Once a negative
inductor current exceeds –400mA typical, the IC will shut
off switch D.
Output Switch Control
Figure 1 shows a simplified diagram of how the four
internal switches are connected to the inductor, V
IN
, V
OUT
and GND. Figure 2 shows the regions of operation for the
LTC3443 as a function of the internal control voltage, V
CI
.
The V
CI
voltage is a level shifted voltage from the output of
the error amp (V
C
pin) (see Figure 5). The output switches
are properly phased so the transfer between operation
modes is continuous, filtered and transparent to the user.
When V
IN
approaches V
OUT
the Buck/Boost region is
reached where the conduction time of the four switch
region is typically 150ns. Referring to Figures 1 and 2, the
various regions of operation will now be described.
Buck Region (V
IN
> V
OUT
)
Switch D is always on and switch C is always off during this
mode. When the internal control voltage, V
CI
, is above
voltage V1, output A begins to switch. During the off time
of switch A, synchronous switch B turns on for the
remainder of the time. Switches A and B will alternate
