ltc3528-2 Linear Technology Corporation, ltc3528-2 Datasheet - Page 10

• Manufacturer Part Number: ltc3528-2
• Description: Ltc3528-2 - 1a, 2mhz Synchronous Step-up Dc/dc Converter In 2mm ? 3mm Dfn
• Manufacturer: Linear Technology Corporation

LTC3528-2
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
via FB from 1.6V to 5.25V.
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
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
P-channel MOSFET synchronous rectifier is only enabled
when V
OPERATION
10
V
OUT
OUT
= 1.20V • 1+
> (V
OUT
IN
to ground programs the output voltage
+ 0.24V).

 
(Refer to Block Diagram)
R2
R1

 
OUT
is close to V
OUT
IN
, the
falls
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
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
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
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
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
OUT
. The output disconnect feature also allows V
IN
. The load at which Burst Mode
SW
(capacitance
OUT
to go to
35282f
OUT