Datasheet

Dropout_Voltage = V
OUT
x
T
OSC
- T
OFF(max)
T
OFF(max)
C
RAMP
=
R
S
x A
g
m
x L
LM25088
LM25088-Q1
www.ti.com SNVS609H –DECEMBER 2008–REVISED MARCH 2013
The sample & hold DC level illustrated in Figure 17 is derived from a measurement of the re-circulating (or free-
wheeling) diode current. The diode current flows through the current sense resistor connected between the CS
and CSG pins. The voltage across the sense resistor is sampled and held just prior to the onset of the next
conduction interval of the buck switch. The diode current sensing and sample & hold provide the DC level for the
reconstructed current signal. The positive slope inductor current ramp is emulated by an external capacitor
connected from the RAMP pin to GND and an internal voltage controlled current source. The ramp current
source that emulates the inductor current is a function of the VIN and VOUT voltages per the following equation:
I
RAMP
= 5 µA/V x (VIN x VOUT) + 25 µA (2)
Proper selection of the RAMP capacitor depends upon the selected value of the output inductor and the current
sense resistor (R
S
). For proper current emulation, the DC sample & hold value and the ramp amplitude must
have the same dependence on the load current. That is:
where
• 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 section.
Dropout Voltage Reduction
The LM25088 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
and T
OSC
are the oscillator frequency and oscillator period, respectively (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 LM25088 controller is a good trade-off between these two requirements. Thus the
LM25088 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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