LTC3535_15 Datasheet, PDF (10/16 Pages) Linear Technology – Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter

LTC3535
OPERATION ^{(Refer to Block Diagram)}
the test mode being engaged. Refer to Figure 1 for two
possible implementations.
independent of input or output voltage, unless V_OUT falls
below 0.7V, in which case the current limit is cut in half.
LTC3535
4M
±30%
SHDN
R
1M
V_IN
V_CNTRL
LTC3535
4M
±30%
SHDN
3535 F01
ZETEX
ZC2811E
R > (V_CNTRL/V_IN + 0.4) – 1)MΩ
1M
V_CNTRL
Figure 1. Recommended Shutdown Circuits When
Driving SHDN Above V_IN
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,
10
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.
Output Disconnect
The LTC3535 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 allows for inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output dis-
connect, there must not be an external Schottky diode
connected between SW and V_OUT. The output disconnect
feature also allows V_OUT to be pulled high, without any
reverse current into a battery connected to V_IN.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3535 will
go into thermal shutdown. All switches will be off and
the soft-start capacitor will be discharged. The device
will be enabled again when the die temperature drops by
about 15°C.
3535fa