Datasheet

LTC3530
7
3530fb
The LTC3530 provides high effi ciency, low noise power
for a wide variety of handheld electronic devices. 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 V
C
determines the output duty cycle of the switches. Since
V
C
is a fi ltered signal, it provides rejection of frequencies
from well below the switching frequency. The low R
DS(ON)
,
low gate charge synchronous switches provide high fre-
quency pulse width modulation control at high effi ciency.
High effi ciency is achieved at light loads when Burst Mode
operation is entered and the LTC3530’s quiescent current
drops to a low 40μA.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is programmed by an external
resistor from R
T
to ground, according to the following
equation:
f
(kHz)
=
33,170
R
T(k)
Error Amp
The error amplifi er is a voltage mode amplifi er. The loop
compensation components are confi gured around the
amplifi er (from FB to V
C
) to obtain stability of the converter.
For improved bandwidth, an additional R-C feed-forward
network can be placed across the upper feedback divider
resistor. The voltage on SHDN/SS clamps the error amp
output, V
C
, to provide a soft-start function.
Internal Current Limit
There are two different current limit circuits in the LTC3530.
Each has internally fi xed thresholds which vary inversely
with V
IN
.
The fi rst circuit is a high speed peak current limit compara-
tor that will shut off switch A once the current exceeds
2.5A typical. The delay to output of this comparator is
typically 50ns.
A second amplifi er will source current out of FB to drop
the output voltage once the peak input current exceeds
2A typical. This method provides a closed loop means of
clamping the input current. During conditions where V
OUT
is near ground, such as during a short-circuit or during
startup, this threshold is cut to 670mA (typ), providing a
foldback feature. For this current limit feature to be most
effective, the Thevenin resistance from FB to ground should
be greater than 100kΩ.
Reverse Current Limit
During fi xed frequency operation, the LTC3530 operates
in forced continuous conduction mode. The reverse cur-
rent limit amplifi er monitors the inductor current from
the output through switch D. Once the negative inductor
current exceeds 640mA typical, the LTC3530 will shut off
switch D.
Four-Switch Control
Figure 1 shows a simplifi ed diagram of how the four in-
ternal switches are connected to the inductor, V
IN
, V
OUT
and GND. Figure 2 shows the regions of operation for the
LTC3530 as a function of the internal control voltage, V
CI
.
3
SW1
4
SW2
PMOS A
NMOS B
7
V
IN
PMOS D
NMOS C
3530 F01
6
V
OUT
85%
D
MAX
BOOST
D
MIN
BOOST
D
MAX
BUCK
DUTY
CYCLE
0%
V4 (≈1.5V)
V3 (≈1.15V)
BOOST REGION
BUCK REGION
BUCK/BOOST REGION
V2 (≈1V)
V1 (≈0.7V)
3530 F02
A ON, B OFF
PWM CD SWITCHES
D ON, C OFF
PWM AB SWITCHES
FOUR SWITCH PWM
INTERNAL
CONTROL
VOLTAGE, V
CI
Figure 1. Simplifi ed Diagram of Output Switches Figure 2. Switch Control vs Internal Control Voltage, V
CI
OPERATION