LTC3534
9
3534fb
For more information www.linear.com/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 V
C
determines the output duty cycle of the
switches. Since V
C
is a filtered signal, it provides rejection
of frequencies 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. High efficiency is achieved at light loads
when Burst Mode operation is invoked and the LTC3534’s
quiescent current drops to a mere 25µA.
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 V
C
) 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, V
C
, 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.
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 V
OUT
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.
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, PV
IN
, V
OUT
, PGND1
and PGND2. Figure 2 shows the regions of operation for
the LTC3534 as a function of the internal control voltage,
V
CI
. Dependent on the magnitude of V
CI
, the LTC3534 will
operate in buck, buck-boost or boost mode. V
CI
is a level
5
SW1
6
SW2L1
PMOS A
NMOS B
12
IN
PMOS D
NMOS C
3534 F01
11
4 7
OUT
PGND1 PGND2
85%
D
MAX
BOOST
D
MIN
BOOST
D
MAX
BUCK
DUTY
CYCLE
0%
V4 (∼1.2V)
INTERNAL
CONTROL
VOLTAGE, V
CI
V3 (∼720mV)
BOOST REGION
BUCK REGION
BUCK-BOOST REGION
V2 (∼640mV)
V1 (∼100mV)
3534 F02
A ON, B OFF
PWM C AND D
SWITCHES
D ON, C OFF
PWM A AND B
SWITCHES
FOUR SWITCH PWM
Figure 1. Simplified Diagram of Output Switches Figure 2. Switch Control vs Internal Control Voltage, V
CI
