ltc3440 Linear Technology Corporation, ltc3440 Datasheet - Page 8

• Manufacturer Part Number: ltc3440
• Description: Micropower Synchronous Buck-boost Dc/dc Converter
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC3440
The LTC3440 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
Since the V
frequencies from well below the switching frequency. The
low R
vide 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% at 600kHz.
High efficiency is achieved at light loads when Burst Mode
operation is entered and when the IC’s quiescent current
is a low 25μA.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is user programmable and is
set through a resistor from the R
An internally trimmed timing capacitor resides inside the
IC. 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 between 100ns and 2μs is applied. The oscillator R
component value required is given by:
where f
For example to achieve a 1.2MHz synchronized switching
frequency the applied clock frequency to the MODE/SYNC
pin is set to 2.4MHz and the timing resistor, R
66.5k (closest 1% value).
8
C
R
f
pin determines the output duty cycle of the switches.
=
T
DS(ON)
⎛
⎜
⎝
=
SW
6 10
R
8 10
e
T
•
= desired synchronized switching frequency.
C
f
SW
, low gate charge synchronous switches pro-
pin is a filtered signal, it provides rejection of
⎞
⎟
⎠
10
Hz
U
T
pin to ground where:
T
, is set to
T
Error Amp
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier to provide loop compensation for the converter.
The SHDN/SS pin will clamp the error amp output, V
provide a soft-start function.
Supply Current Limit
The current limit amplifier will shut PMOS switch A off
once the current exceeds 2.7A typical. The current ampli-
fier 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
and GND. Figure 2 shows the regions of operation for the
LTC3440 as a function of the internal control voltage, V
The V
the error amp (V
are properly phased so the transfer between operation
modes is continuous, filtered and transparent to the user.
When V
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.
CI
voltage is a level shifted voltage from the output of
Figure 1. Simplified Diagram of Output Switches
IN
approaches V
V
7
IN
PMOS A
NMOS B
C
SW1
pin) (see Figure 5). The output switches
3
OUT
SW2
the Buck/Boost region is
4
V
OUT
6
3440 F01
PMOS D
NMOS C
V
OUT
IN
, V
C
3440fb
OUT
, to
CI
.