LTC3400-1 Datasheet, PDF (5/12 Pages) Linear Technology – 600mA, 1.2MHz Micropower Synchronous Boost Converter in ThinSOT

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OPERATIO
The LTC3400-1 is a1.2MHz, synchronous boost converter
housed in a 6-lead ThinSOT package. Able to operate from
an input voltage below 1V, the device features fixed
frequency, current mode PWM control for exceptional line
and load regulation. With its low R_DS(ON) and gate charge
internal MOSFET switches, the device maintains high
efficiency over a wide range of load current. Detailed
descriptions of the three distinct operating modes follow.
Operation can be best understood by referring to the Block
Diagram.
Low Voltage Start-Up
The LTC3400-1 will start up at a typical V_IN voltage of
0.85V or higher. The low voltage start-up circuitry controls
the internal NMOS switch up to a maximum peak inductor
current of 850mA (typ), with an approximate 1.5µs off-
time during start-up, allowing the device to start up into an
output load. Once V_OUT exceeds 2.3V, the start-up cir-
cuitry is disabled and normal fixed frequency PWM opera-
tion is initiated. In this mode, the LTC3400-1 operates
independent of V_IN, allowing extended operating time as
the battery can droop to several tenths of a volt without
affecting output voltage regulation. The limiting factor for
the application becomes the ability of the battery to supply
sufficient energy to the output.
Low Noise Fixed Frequency Operation
Oscillator: The frequency of operation is internally set to
1.2MHz.
Error Amp: The error amplifier is an internally compensated
transconductance type (current output) with a transconduc-
tance (g_m) = 33 microsiemens. The internal 1.23V reference
voltage is compared to the voltage at the FB pin to generate
an error signal at the output of the error amplifier. A volt-
age divider from V_OUT to ground programs the output
voltage via FB from 2.5V to 5V using the equation:
V_OUT = 1.23V • [1 + (R1/R2)]
Current Sensing: A signal representing NMOS switch
current is summed with the slope compensator. The
summed signal is compared to the error amplifier output
LTC3400-1
to provide a peak current control command for the PWM.
Peak switch current is limited to approximately 850mA
independent of input or output voltage. The current signal
is blanked for 40ns to enhance noise rejection.
Zero Current Comparator: The zero current comparator
monitors the inductor current to the output and shuts off
the synchronous rectifier once this current reduces to ap-
proximately 20mA. This prevents the inductor current from
reversing in polarity improving efficiency at light loads.
Antiringing Control: The antiringing control circuitry pre-
vents high frequency ringing of the SW pin as the inductor
current goes to zero by damping the resonant circuit
formed by L and C_SW (capacitance on SW pin).
Burst Mode Operation
Portable devices frequently spend extended time in low
power or standby mode, only switching to high power
drain when specific functions are enabled. In order to
improve battery life in these types of products, high power
converter efficiency needs to be maintained over a wide
output power range. In addition to its high efficiency at
moderate and heavy loads, the LTC3400-1 includes auto-
matic Burst Mode operation that improves efficiency of
the power converter at light loads. Burst mode operation
is initiated if the output load current falls below an
internally programmed threshold (see Typical Perfor-
mance graph, Output Load Burst Mode Threshold vs V_IN).
Once initiated, the Burst Mode operation circuitry shuts
down most of the device, only keeping alive the circuitry
required to monitor the output voltage. This is referred to
as the sleep state. In sleep, the LTC3400-1 draws only
19µA from the output capacitor, greatly enhancing effi-
ciency. When the output voltage has drooped approxi-
mately 1% from nominal, the LTC3400-1 wakes up and
commences normal PWM operation. The output capacitor
recharges and causes the LTC3400-1 to reenter sleep if
the output load remains less than the sleep threshold. The
frequency of this intermittent PWM or burst operation is
proportional to load current; that is, as the load current
drops further below the burst threshold, the LTC3400-1
turns on less frequently. When the load current increases
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