Datasheet

LIM_U
LIM_U
1.23
Upper range: R = 20000
I
æ ö
´ç ÷
ç ÷
è ø
LIM_L
LIM_L
1.23
Lower range: R = 5400
I
æ ö
´ç ÷
ç ÷
è ø
+
+
470pF
Ω
Ω
Ω
Ω
Ω
Ω
TPS62750, TPS62751
SLVS955A –JULY 2009–REVISED SEPTEMBER 2009 ................................................................................................................................................. www.ti.com
(7)
(8)
Examples of different input current limit values selectable are given in the table below:
AVERAGE INPUT RESISTOR VALUE
CURRENT REQUIRED
50 mA 132.8 K
100 mA 66.5 K
400 mA 61.5 K
500 mA 49.2 K
600 mA 41 K
700 mA 35.1 K
CHECKING LOOP STABILITY
The first step of circuit and stability evaluation is to look from a steady-state perspective at the following signals:
• Switching node, SW
• Inductor current, I
L
• Output ripple voltage, V
O(AC)
These are the basic signals that need to be measured when evaluating a switching converter. When the
switching waveform shows large duty cycle jitter or the output voltage or inductor current shows oscillations, the
regulation loop may be unstable. This is often a result of board layout and/or L-C combination.
As a next step in the evaluation of the regulation loop, the load transient response is tested. The time between
the application of the load transient and the turn on of the P-channel MOSFET, the output capacitor must supply
all of the current required by the load. V
O
immediately shifts by an amount equal to ΔI
(LOAD)
× ESR, where ESR is
the effective series resistance of C
O
. ΔI
(LOAD)
begins to charge or discharge C
O
generating a feedback error
signal used by the regulator to return V
O
to its steady-state value. The results are most easily interpreted when
the device operates in PWM mode.
During this recovery time, V
O
can be monitored for settling time, overshoot or ringing that helps judge the
converter’s stability. Without any ringing, the loop has usually more than 45° of phase margin. Because the
damping factor of the circuitry is directly related to several resistive parameters (e.g., MOSFET R
DSon
) that are
temperature dependant, the loop stability analysis has to be done over the input voltage range, load current
range, and temperature range.
Figure 27. Checking Loop Stability
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