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ADP1822 Datasheet(PDF) 15 Page - Analog Devices

Part No. ADP1822
Description  PWM, Step-Down DC-to-DC Controller with Margining and Tracking
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADP1822 Datasheet(HTML) 15 Page - Analog Devices

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ADP1822
Rev. B | Page 15 of 24
When fESRZ is approximately the same as the switching frequency,
the square-root sum of the squares of the two ripples applies, or
[]
()()
2
2
8
Δ
)
(
Δ
Δ
+
SW
OUT
L
L
OUT
f
C
I
ESR
I
V
(6)
The low-side MOSFET does not carry the transition losses but
does carry the inductor current when the high-side MOSFET is
off. For high input and low output voltages, the low-side
MOSFET carries the current most of the time, and therefore to
achieve high efficiency, it is critical to optimize the low-side
MOSFET for low on resistance. In some cases, where the power
loss exceeds the MOSFET rating, or lower resistance is required
than is available in a single MOSFET, connect multiple low-side
MOSFETs in parallel. The equation for low-side MOSFET
power loss is
SELECTING THE MOSFETS
The choice of MOSFET directly affects the dc-to-dc converter
performance. The MOSFET must have low on resistance to
reduce I2R losses and low gate charge to reduce transition losses.
In addition, the MOSFET must have low thermal resistance to
ensure that the power dissipated in the MOSFET does not result
in excessive MOSFET die temperature.
() ( )
IN
OUT
ON
LOAD
LS
V
V
R
I
P
1
2
(11)
where:
P
The high-side MOSFET carries the load current during on time
and carries all the transitions losses of the converter. Typically,
the lower the MOSFET on resistance, the higher the gate charge
and vice versa. Therefore, it is important to choose a high-side
MOSFET that balances the two losses. The conduction loss of
the high-side MOSFET is determined by
LS
is the low-side MOSFET on resistance.
R
is the total on resistance of the low-side MOSFET(s).
ON
If multiple low-side MOSFETs are used in parallel, use the
parallel combination of the on resistances for determining RON
to solve this equation.
() ( )
⎟⎟
⎜⎜
IN
OUT
ON
LOAD
C
V
V
R
I
P
2
(7)
SETTING THE CURRENT LIMIT
The internal current-limit circuit measures the voltage across
the low-side MOSFET to determine the load current. When the
low-side MOSFET current exceeds the current limit, the high-
side MOSFET is not allowed to turn on until the current drops
below the current limit.
where:
PC = conduction power loss.
R
= MOSFET on resistance.
ON
The gate-charging loss is approximated by
(
)( )( )
SW
G
PVCC
T
f
Q
V
P ≅
(8)
The current limit is set through the current-limit resistor, RCL.
The current sense pin, CSL, sources 50 μA through RCL. This
creates an offset voltage of resistance of RCL multiplied by the
50 μA CSL current. When the low-side MOSFET voltage is
equal to or greater than the offset voltage, the ADP1822 is in
current limit mode and prevents additional on-time cycles.
where:
PT = gate-charging loss power.
VPVCC = gate driver supply voltage.
QG = MOSFET total gate charge.
fSW = converter switching frequency.
Choose the current limit resistor by the equation
The high-side MOSFET transition loss is approximated by
()
2
SW
F
R
LOAD
IN
SW
f
t
t
I
V
P
×
+
×
×
=
(9)
(
)
(
)
μA
42
ONWC
LPK
CL
R
I
R
=
(12)
where:
ILPK is the peak inductor current.
R
where:
P
ONWC
is the worst-case (maximum) low-side MOSFET on
resistance.
SW
= high-side MOSFET switching loss power.
tR = MOSFET rise time.
tF = MOSFET fall time.
The total power dissipation of the high-side MOSFET is the
sum of all the previous losses, or
( ) ( ) ( )
SW
T
C
HS
P
P
P
P
+
+
(10)
The worst-case, low-side MOSFET on resistance can be found
in the MOSFET data sheet. Note that MOSFETs typically
increase on resistance with increasing die temperature. To
determine the worst-case MOSFET on resistance, calculate the
worst-case MOSFET temperature (based on the MOSFET
power loss) and multiply by the ratio between the typical
on resistance at that temperature and the on resistance at 25°C
as listed in the MOSFET data sheet.
where P is the total high-side MOSFET power loss.
HS


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