Datasheet

Dropout_Voltage = V
OUT
x
T
OSC
- T
OFF(max)
T
OFF(max)
LM5088, LM5088-Q1
SNVS600H –DECEMBER 2008–REVISED MARCH 2013 www.ti.com
• g
m
is the ramp current generator transconductance (5 µA/V)
• A is the gain of the current sense amplifier (10V/V) (3)
The RAMP capacitor should connected directly to the RAMP and GND pins of the IC.
For duty cycles greater than 50%, peak current mode control circuits are subject to sub-harmonic oscillation.
Sub-harmonic oscillation is normally characterized by alternating wide and narrow pulses at the SW pin. Adding
a fixed slope voltage ramp (slope compensation) to the current sense signal prevents this oscillation. The 25 µA
offset current supplied by the emulated current source provides a fixed slope to the ramp signal. In some high
output voltage, high duty cycles applications; additional slope compensation may be required. In these
applications, a pull-up resistor may be added between the RAMP and VCC pins to increase the ramp slope
compensation. A formula to configure pull-up resistor is shown in Applications Information.
Dropout Voltage Reduction
The LM5088 features unique circuitry to reduce the dropout voltage. Dropout voltage is defined as the difference
between the minimum input voltage to maintain regulation and the output voltage (VIN
min
- Vout). Dropout voltage
thus determines the lowest input voltage at which the converter maintains regulation. In a buck converter,
dropout voltage primarily depends upon the maximum duty cycle. The maximum duty cycle is dependant on the
oscillator frequency and minimum off-time.
An approximation for the dropout voltage is:
where
• T
OSC
= 1/f
SW
• T
OFF (max)
is the forced off-time (280 ns typical, 365 ns maximum)
• f
SW
is the oscillator frequency
• T
OSC
is the oscillator period (4)
From the above equation, it can be seen that for a given output voltage, reducing the dropout voltage requires
either reducing the forced off-time or oscillator frequency (1/T
OSC
). The forced off-time is limited by the time
required to replenish the bootstrap capacitor and time required to sample the re-circulating diode current. The
365 ns forced off-time of the LM5088 controller is a good trade-off between these two requirements. Thus the
LM5088 reduces dropout voltage by dynamically decreasing the operating frequency during dropout. The
Dynamic Frequency Control (DFC) is achieved using a dropout monitor, which detects a dropout condition and
reduces the operating frequency. The operating frequency will continue to decrease with decreasing input
voltage until the frequency falls to the minimum value set by the DFC circuitry.
f
SW(minDFC)
≊ 1/3 x f
SW(nominal)
(5)
If the VIN voltage continues to fall below this point, output regulation can no longer be maintained. The oscillator
frequency will revert back to the nominal operating frequency set by the RT resistor when the input voltage
increases above the dropout range. DFC circuitry does not affect the PWM during normal operating conditions.
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