LTC3534 Просмотр технического описания (PDF) - Linear Technology
Номер в каталоге
Компоненты Описание
Список матч
LTC3534 Datasheet PDF : 20 Pages
| |||
LTC3534
Operation
The LTC3534 provides high efficiency, low noise power
for a wide variety of handheld electronic devices. Linear
Technology’s proprietary topology allows input voltages
above, below or equal to the output voltage by properly
phasing the output switches. The error amplifier 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 frequency pulse width modulation control at
high efficiency. High efficiency is achieved at light loads
when Burst Mode operation is invoked and the LTC3534’s
quiescent current drops to a mere 25µA.
The first circuit is an average current limit amplifier, sourc-
ing current out of FB to drop the output voltage should
the peak input current exceed 1.8A typical. This method
provides a closed loop means of clamping the input cur-
rent. During conditions where VOUT is near ground, such
as during a short circuit or start-up, this threshold is cut
to 800mA typical, providing a foldback feature. For this
current limit feature to be most effective, the Thevenin
resistance from FB to ground should be greater than 100k.
Should the peak input current exceed 2.6A typical, the
second circuit, a high speed peak current limit compara-
tor, shuts off PMOS switch A. The delay to output of this
comparator is typically 50ns.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is internally set to 1MHz.
Error Amplifier
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 feedforward
network can be placed across the upper feedback divider
resistor. The voltage on RUN/SS clamps the error amplifier
output, VC, to provide a soft-start function.
Supply Current Limits
There are two different supply current limit circuits in the
LTC3534, each having internally fixed thresholds.
PVIN
VOUT
12
11
PMOS A
SW1
L1
SW2
5
6
NMOS B
PMOS D
NMOS C
4
PGND1
7 3534 F01
PGND2
Reverse Current Limit
During fixed frequency operation, the LTC3534 operates
in forced continuous conduction mode. The reverse cur-
rent limit comparator monitors the inductor current from
the output through PMOS switch D. Should this negative
inductor current exceed 500mA typical, the LTC3534
shuts off switch D.
Four-Switch Control
Figure 1 shows a simplified diagram of how the four internal
switches are connected to the inductor, PVIN, VOUT, PGND1
and PGND2. Figure 2 shows the regions of operation for
the LTC3534 as a function of the internal control voltage,
VCI. Dependent on the magnitude of VCI, the LTC3534 will
operate in buck, buck-boost or boost mode. VCI is a level
DUTY
CYCLE
85%
DMAX
BOOST
DMIN
BOOST
DMAX
BUCK
0%
A ON, B OFF
PWM C AND D
SWITCHES
BOOST REGION
FOUR SWITCH PWM
BUCK-BOOST REGION
D ON, C OFF
PWM A AND B
SWITCHES
BUCK REGION
INTERNAL
CONTROL
VOLTAGE, VCI
V4 (∼1.2V)
V3 (∼720mV)
V2 (∼640mV)
V1 (∼100mV)
3534 F02
Figure 1. Simplified Diagram of Output Switches
Figure 2. Switch Control vs Internal Control Voltage, VCI
3534fb
For more information www.linear.com/LTC3534
9
Share Link: 