Datasheet

ADP1851 Data Sheet
Rev. 0 | Page 18 of 24
There is also additional power loss during the time, known as
dead time, between the turn-off of the high-side switch and the
turn-on of the low-side switch, when the body diode of the low-
side MOSFET conducts the output current. The power loss in
the body diode is given by
OSW
DF
BODYDIODE
IftVP ×××=
where:
V
F
is the forward voltage drop of the body diode, typically 0.7 V.
t
D
is the dead time in the ADP1851, typically 25 ns when
driving a medium size MOSFET with input capacitance, C
ISS
,
of approximately 3 nF. The dead time is not fixed. Its effective
value varies with gate drive resistance and C
ISS
; therefore,
P
BODYDIODE
increases in high load current designs and low voltage
designs.
Therefore, the power loss in the low-side MOSFET is
BODYDIODECLSLS
PPP +=
Note that the MOSFET on resistance, R
DSON
, increases with
increasing temperature, with a typical temperature coefficient of
0.4%/°C. The MOSFET junction temperature (T
J
) rise over the
ambient temperature is
T
J
= T
A
+ θ
JA
× P
D
where:
T
A
is the ambient temperature.
θ
JA
is the thermal resistance of the MOSFET package.
P
D
is the total power dissipated in the MOSFET.
LOOP COMPENSATION—VOLTAGE MODE
Set the controller to voltage mode operation by placing a
100 kΩ resistor between DL and PGND. Choose the largest
possible ramp amplitude for the voltage mode below 1.5 V.
The ramp voltage is programmed by a resistor placed between
V
IN
and the RAMP pin as follows:
RAMPSW
IN
RAMP
Vf
V
R
××
−
=
pF100
V2.0
The voltage at the RAMP pin is fixed at 0.2 V, and the current
going into RAMP should be between 10 µA and 160 µA. Make
sure that the following condition is satisfied:
μA160
V2.0
μA10 ≤
−
≤
RAMP
IN
R
V
(3)
For example, with an input voltage of 12 V, R
RAMP
should not be
less than 73.8 kΩ.
Assuming that the LC filter design is complete, the feedback
control system can be compensated. In general, aluminum
electrolytic capacitors have high ESR; however, if several
aluminum electrolytic capacitors are connected in parallel and
produce a low effective ESR, then Type III compensation is
needed. In addition, ceramic capacitors have very low ESR (only
a few milliohms), making Type III compensation a better
choice.
Type III Compensation
Figure 26. Type III Compensation
If the output capacitor ESR zero frequency is greater than one-
half of the crossover frequency, use the Type III compensator as
shown in Figure 26.
Calculate the output LC filter resonant frequency as follows:
LCπ
f
LC
2
1
=
(4)
Choose a crossover frequency that is 1/10 of the switching
frequency:
10
SW
CO
f
f =
(5)
Set the poles and zeros as follows:
SW
P2P1
fff
2
1
==
(6)
I
Z
SWCO
Z2Z1
CR
ff
ff
π 2
1
404
====
(7)
or
I
Z
LC
Z2Z1
CR
f
ff
π 2
1
2
===
(8)
Use the lower zero frequency from Equation 7 or Equation 8.
Calculate the compensation resistor, R
Z
, as follows:
2
LC
IN
CO
Z1
RAMP
TOP
Z
fV
ffVR
R =
(9)
Next, calculate C
I
.
Z1Z
I
fR
C
π
=
2
1
(10)
Because of the finite output current drive of the error amplifier,
C
I
must be less than 10 nF. If it is larger than 10 nF, choose a
larger R
TOP
and recalculate R
Z
and C
I
until C
I
is less than 10 nF.
G
(dB)
PHASE
–90°
–270°
f
Z
f
P
C
HF
C
I
R
Z
R
FF
R
TOP
R
BOT
V
OUT
INTERNAL
V
REF
EA
FB
COMP
–1 SLOPE
–1 SLOPE
C
FF
+1 SLOPE
10595-026