LTC3535 Datasheet, PDF (10/16 Pages) Linear Technology – Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter

LTC3535
OPERATION ^{(Refer to Block Diagram)}
Oscillator
An internal oscillator (independent for each channel) sets
the switching frequency to 1MHz.
Shutdown
Shutdown is accomplished by pulling the SHDN pin below
0.3V and enabled by pulling the SHDN pin above 0.8V. Note
that SHDN can be driven above V_IN or V_OUT, as long as it
is limited to less than the absolute maximum rating.
Error Amplifier
The positive input of the transconductance error amplifier
is internally connected to the 1.195V reference and the
negative input is connected to FB. Clamps limit the mini-
mum and maximum error amp output voltage for improved
large-signal transient response. Power converter control
loop compensation is provided internally. An external
resistive voltage divider from V_OUT to ground programs
the output voltage via FB from 1.5V to 5.25V.
V_OUT
=
1.195V
•
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
Current Sensing
Lossless current sensing converts the peak current signal of
the N-channel MOSFET switch into a voltage that 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 750mA,
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 30mA. 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-ring circuit connects a resistor across the in-
ductor to prevent high frequency ringing on the SW pin
during discontinuous current mode operation. Although
the ringing of the resonant circuit formed by L and C_SW
(capacitance on SW pin) is low energy, it can cause EMI
radiation.
3535f
10