Datasheet

C
O
=
('V + V
OUT
)
2
- V
OUT
2
'I
PP
I
O
+
2
L x
2
VCC
R
RAMP
C
RAMP
RAMP
R
RAMP
=
V
VCC
- V
RAMP
I
OS
- 25 PA
C
RAMP
=
5 PA/V x 6.8 PH
10V/V x 10 m:
= 340 pF
C
RAMP
=
g
m
x L
A x R
S
LM25088
LM25088-Q1
SNVS609H –DECEMBER 2008–REVISED MARCH 2013 www.ti.com
where
• L is the value of the output inductor
• gm is the ramp generator transconductance (5 µA/V)
• A is the current sense amplifier gain (10V/V) (12)
For the current design example, the ramp capacitor is calculated as:
(13)
The next lowest standard value 270 pF was selected for C
RAMP
. An NPO capacitor with 5% or better tolerance is
recommended. It should be noted that selecting a capacitor value lower than the calculated value will increase
the slope compensation. Furthermore, selecting a ramp capacitor substantially lower or higher than the
calculated value will also result in incorrect PWM operation.
For VOUT > 5V, internal slope compensation provided by the LM25088 may not be adequate for certain
operating conditions especially at low input voltages. A pull-up resistor may be added from VCC to RAMP the pin
to increase the slope compensation. Optimal slope compensation current may be calculated from
I
OS
= V
OUT
x 5 µA/V (14)
and R
RAMP
is given by
(15)
Figure 24. Additional Slope Compensation for VOUT > 5V
OUTPUT CAPACITORS
The output capacitors smooth the inductor current ripple and provide a source of charge for load transient
conditions. The output capacitor selection is primarily dictated by the following specifications:
1. Steady-state output peak-peak ripple (ΔV
PK-PK
)
2. Output voltage deviation during transient condition (ΔV
Transient
)
For the 5V output design example, ΔV
PK-PK
= 50 mV (1% of VOUT) and ΔT
Transient
= 100 mV (2% of VOUT) was
chosen. The magnitude of output ripple primarily depends on ESR of the capacitors while load transient voltage
deviation depends both on the output capacitance and ESR.
When a full load is suddenly removed from the output, the output capacitor must be large enough to prevent the
inductor energy to raise the output voltage above the specified maximum voltage. In other words, the output
capacitor must be large enough to absorb the inductor’s maximum stored energy. Equating, the stored energy
equations of both the inductor and the output capacitor it can be shown that:
(16)
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