LM2641MTC-ADJ Datasheet, PDF (14/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
SNVS040B – JANUARY 2000 – REVISED APRIL 2013
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M_COMP = M_{CS AMP} (max) = N X R_SENSE X V_OUT (max) / L
Where:
M_COMP is the slope of the compensation ramp.
M_{CS AMP} (max) is the maximum downward slope of the voltage at the output of the current sense amplifier.
N is the gain of the current sense amplifier.
R_SENSE is the value of the current sense resistor.
V_OUT (max) is the maximum output voltage.
L is the inductance of the output inductor.
It is important to note that since the value R_SENSE appears in the numerator and L is in the denominator, these
two values may be increased or decreased at the same ratio without changing the slope.
At higher values of load current, a lower value R_SENSE will be selected. The inductance value for the output
inductor should be decreased by the same percentage to maintain correct ramp compensation.
APPLICATION INFORMATION
Improved Transient Response
If the output voltage falls below 97% of the nominal value, the low-voltage regulation (LREG) comparator will
activate logic which turns ON the high-side FET switch continuously until the output returns to nominal. The low-
side FET switch is held OFF during this time.
This action will improve transient response since it bypasses the error amplifier and PWM comparator, forcing
the high-side switch ON until the output returns to nominal. This feature is disabled during start-up.
Boost High-Side Gate Drive
A “flying” bootstrap capacitor is used to generate the gate drive voltage used for the high-side FET switch. This
bootstrap capacitor is charged up to about 5V using an internal supply rail and diode when ever the low-side FET
switch is ON. When the high-side FET switch turns ON, the Source is pulled up near the input voltage. The
voltage across the bootstrap capacitor boosts up the gate drive voltage, ensuring that the Gate is driven at least
4.3V higher than the Source.
Reference
The internal bandgap reference is used to generate a 2.5V reference voltage which is connected to the REF pin.
The specified tolerance of the REF voltage is ±2% over the full operating temperature range, as long as the
current drawn is ≤ 5 mA.
A bypass capacitor on the REF pin is not required, but may be used to reduce noise.
5V LIN Output
The LM2641 contains a built-in 5V/50 mA LDO regulator whose output is connected to the LIN pin. Since this is
an LDO regulator, it does require an external capacitor to maintain stability. The minimum amount of capacitance
required for stability is 4.7 μF, with ESR in the range of about 100 mΩ to 3Ω. A good quality solid Tantalum
capacitor is recommended (ceramics can not be used because the ESR is too low). If cold temperature operation
is required, a capacitor must be selected which has an ESR that is in the stable range over the entire operating
temperature range of the application.
Since the current limit for this LDO regulator is set at about 85 mA, it can be used at load currents up to about 50
mA (assuming total IC power dissipation does not exceed the maximum value).
Ensured specifications are provided for worst-case values of V_LIN over the full operating temperature range for
load currents up to 25mA (see Electrical Characteristics). To estimate how the V_LIN output voltage changes when
going from I_LIN = 25mA to I_LIN = 50mA, a change in V_LIN of about −30mV should be expected due to loading
(typical value only, not specified). This decrease in V_LIN is linear with increasing load current.
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