LTC3528-2
10
35282fb
Error Amplifier
The error amplifier is a transconductance type. The nonin-
verting input is internally connected to the 1.20V reference
and the inverting input is connected to FB. Clamps limit
the minimum and maximum error amp output voltage for
improved large-signal transient response. Power converter
control loop compensation is provided internally. A voltage
divider from V
OUT
to ground programs the output voltage
via FB from 1.6V
to 5.25V.
V
OUT
= 1.20V • 1+
R2
R1
Current Sensing
Lossless current sensing converts the peak current signal
of the N-channel MOSFET switch into a voltage which
is summed with the internal slope compensation. The
summed signal is compared to the error amplifier output
to provide a peak current control command for the PWM.
Current Limit
The current limit comparator shuts off the N-channel MOS-
FET switch once its threshold is reached. The current
limit
comparator delay to output is typically 60ns. Peak switch
current is limited to approximately 1.5A, independent of
input or output voltage, unless V
OUT
falls below 0.7V, in
which case the current limit is cut in half.
Zero Current Comparator
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifier
when this current reduces to approximately 20mA. This
prevents
the inductor current from reversing in polarity,
improving efficiency at light loads.
Synchronous Rectifier
To control inrush current and to prevent the inductor
current from running away when V
OUT
is close to V
IN
, the
P-channel MOSFET synchronous rectifier is only enabled
when V
OUT
> (V
IN
+ 0.24V).
Anti-Ringing Control
The anti-ringing control connects a resistor across the
inductor to prevent high frequency ringing on the
SW pin
during discontinuous current mode operation. The ringing
of the resonant circuit formed by L and C
SW
(capacitance
on SW pin) is low energy, but can cause EMI radiation.
Output Disconnect
The LTC3528-2 is designed to allow true output discon-
nect by eliminating body diode conduction of the internal
P-channel MOSFET rectifier. This allows for V
OUT
to go to
zero volts during shutdown, drawing
no current from the
input source. It also enables inrush current limiting at turn-
on, minimizing surge currents seen by the input supply.
Note that to obtain the advantages of output disconnect,
a Schottky diode cannot be connected between SW and
V
OUT
. The output disconnect feature also allows V
OUT
to be forced above the programmed regulation voltage,
without any reverse current into the input power source.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3528-2
enters thermal shutdown. All switches will be turned off
and the soft-start capacitor will be discharged. The device
will be enabled again when the die temperature drops by
approximately 15°C.
Burst Mode OPERATION
The LTC3528-2 will automatically enter Burst Mode op-
eration at light load current and return to fixed frequency
PWM mode when the load
increases. Refer to the Typical
Performance Characteristics to see the output load Burst
Mode threshold vs V
IN
. The load at which Burst Mode
operation is entered can be changed by adjusting the
inductor value. Raising the inductor value will lower the
load current at which Burst Mode operation is entered.
In Burst Mode operation, the LTC3528-2 continues switch-
ing at a fixed frequency of 2MHz, using
the same error
amplifier and loop compensation for peak current mode
control. This control method minimizes output transients
OPERATION
(Refer to Block Diagram)
