Datasheet

Rfb2 =
Rfb
1
=
- 1
V
OUT
0.8V
249 k:
- 1
1.2V
0.8V
= 499 k:
Rfb1 =
1
2SCc1f
LC
=
1
2S47 pF 16.8 kHz
= 200 k:
Rc1 =
1
2SCc1f
ESR
=
1
2S47 pF 885 kHz
= 3.8 k:
Cc1(pF) = D
Lf(PH)Co1(PF)
V
IN
(V)
f
loop
(kHz)
Cc1 = 0.038
1.5 60
5.5
75 = 47 pF
f
LC
#
LfCo1
1
2S
=
1.5 PH 60 PF
1
2S
= 16.8 kHz
f
ESR
#
1
2SR
ESR
Co1
=
1
2S3 m: 60 PF
= 885 kHz
Rtrk1 =
10 k:
(V
TRACK
- 1.0V)
, Simultaneous Tracking
0.8V
(V
OUT
± 0.8V)
, Ratiometric Tracking
10 k:
Component Selection www.ti.com
7.5 Tracking Resistors
If simultaneous (equal voltage ramp rates) or ratiometric (equal soft-start times) tracking to an external
supply voltage, V
TRACK
, is required, the user can remove the soft-start capacitor and replace with the
appropriate value lower tracking resistor. The upper resistor, Rtrk, is 10 kΩ. Thus, the lower tracking
resistor value, designated Rtrk1, is governed by
(11)
7.6 Enable Resistors
The LM2854 has an enable pin with an on-board 100 kΩ pull-up resistor, designated Ren1. Thus, the
associated PCB terminal, labeled EN, can be left open circuit if desired. Alternatively, the enable function
can be configured to increase the input UVLO level by setting up a resistor divider from VIN using
resistors Ren1 and Ren2. The board is supplied with Ren2 not assembled.
7.7 Feedback and Compensation Components
The voltage loop crossover frequency, f
loop
, is usually selected between one tenth and one fifth of the
switching frequency
0.1 f
SW
≤ f
loop
≤ 0.2 f
SW
(12)
The complex double pole related to the LC output filter and zero due to the output capacitor ESR are as
follows
(13)
A simple solution for the required external compensation capacitor, C
COMP
, with type III voltage mode
control can be expressed as follows where the constant is nominally 0.038 for the 500 kHz option.
(14)
The upper feedback resistor, Rfb1, is selected to provide adequate mid-band gain and to locate a zero at
or below the LC pole frequency. The series resistor, Rc1, is selected to locate a pole at the ESR zero
frequency. Thus
(15)
Rfb1 and Rc1 are chosen as 249 kΩ and 1 kΩ in the evaluation board. With Rfb1 defined based on the
voltage loop requirements, Rfb2, the lower feedback resistor, is then selected for the desired output
voltage by
(16)
Note that Rfb2 has no impact on the control loop from an AC standpoint since the FB pin is the input to an
op-amp type error amplifier and effectively at AC ground. Hence, the control loop can be designed
irrespective of output voltage level. The only caveat here is the necessary derating of the output
capacitance with applied voltage.
6
AN-1786 LM2854 500 kHz Buck Regulator Evaluation Board SNVA323B–March 2008–Revised May 2013
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