Datasheet

LTC3528/LTC3528B
9
3528fd
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
MOSFET switch once its threshold is reached. The cur-
rent 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/LTC3528B is designed to allow true output
disconnect 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 a battery on V
IN
.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3528/
LTC3528B will enter 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.
OPERATION
(Refer to Block Diagram)