LTC3530 データシートの表示（PDF） - Linear Technology
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LTC3530 Datasheet PDF : 16 Pages
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LTC3530
OPERATION
The LTC3530 provides high efficiency, 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 VC
determines the output duty cycle of the switches. Since
VC is a filtered signal, it provides rejection of frequencies
from well below the switching frequency. The low RDS(ON),
low gate charge synchronous switches provide high fre-
quency pulse width modulation control at high efficiency.
High efficiency 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 RT to ground, according to the following
equation:
f (kHz)
=
33,170
RT(k)
Error Amp
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier (from FB to VC) 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, VC, to provide a soft-start function.
VIN
7
PMOS A
SW1
3
NMOS B
VOUT
6
PMOS D
SW2
4
NMOS C
3530 F01
Figure 1. Simplified Diagram of Output Switches
Internal Current Limit
There are two different current limit circuits in the LTC3530.
Each has internally fixed thresholds which vary inversely
with VIN.
The first 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 amplifier 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 VOUT
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 fixed frequency operation, the LTC3530 operates
in forced continuous conduction mode. The reverse cur-
rent limit amplifier 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 simplified diagram of how the four in-
ternal switches are connected to the inductor, VIN, VOUT
and GND. Figure 2 shows the regions of operation for the
LTC3530 as a function of the internal control voltage, VCI.
85%
DMAX
BOOST
DMIN
BOOST
DMAX
BUCK
A ON, B OFF
PWM CD SWITCHES
BOOST REGION
FOUR SWITCH PWM
BUCK/BOOST REGION
D ON, C OFF
PWM AB SWITCHES
BUCK REGION
0%
V4 (≈1.5V)
V3 (≈1.15V)
V2 (≈1V)
V1 (≈0.7V)
DUTY
CYCLE
INTERNAL
3530 F02 CONTROL
VOLTAGE, VCI
Figure 2. Switch Control vs Internal Control Voltage, VCI
3530fb
7
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