DS6550A Datasheet, PDF (14/16 Pages) Richtek Technology Corporation – Synchronous DC-DC Step-Down Controller with 5V LDO

RT6550A
Output Inductor Selection
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as shown
below :
L

t_ON  (V_IN  V_OUT
LIR I_LOAD(MAX)
)
where LIR is the ratio of the peak-to-peak ripple current to
the average inductor current.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice, although powdered iron is
inexpensive and can work well at 200kHz. The core must
be large enough not to saturate at the peak inductor
current, I_PEAK :
I_PEAK = I_LOAD(MAX) + [ (LIR / 2) x I_LOAD(MAX) ]
The calculation above shall serve as a general reference.
To further improve transient response, the output
inductance can be further reduced. Of course, besides
the inductor, the output capacitor should also be
considered when improving transient response.
Output Capacitor Selection
The capacitor value and ESR determine the amount of
output voltage ripple and load transient response. Thus,
the capacitor value must be greater than the largest value
calculated from the equations below :
V_SAG

(I_LOAD )² L  (t_ON + t_OFF(MIN) )
2 C_OUT  V_IN  t_ON  V_OUTx (t_ON + t_OFF(MIN) )
V_SOAR

(I_LOAD )² L
2  C_OUT  V_OUTx
V_PP
 LIR I_LOAD(MAX)  ^ESR +

1
8
 C_OUT

f


where V_SAG and V_SOAR are the allowable amount of
undershoot and overshoot voltage during load transient,
V_p-p is the output ripple voltage, and t_OFF(MIN) is the
minimum off-time.
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14
Thermal Considerations
The junction temperature should never exceed the
absolute maximum junction temperature T_J(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
dissipation depends on the thermal resistance of the IC
package, the PCB layout, the rate of surrounding airflow,
and the difference between the junction and ambient
temperatures. The maximum power dissipation can be
calculated using the following formula :
P_D(MAX) = (T_J(MAX) − T_A) / θ_JA
where T_J(MAX) is the maximum junction temperature, T_A is
the ambient temperature, and θ_JA is the junction-to-ambient
thermal resistance.
For continuous operation, the maximum operating junction
temperature indicated under Recommended Operating
Conditions is 125°C. The junction-to-ambient thermal
resistance, θ_JA, is highly package dependent. For a
WDFN-10L 3x3 package, the thermal resistance, θ_JA, is
30.5°C/W on a standard JEDEC 51-7 high effective-thermal-
conductivity four-layer test board. The maximum power
dissipation at T_A = 25°C can be calculated as below :
P_D(MAX) = (125°C − 25°C) / (30.5°C/W) = 3.27W for a
WDFN-10L 3x3 package.
The maximum power dissipation depends on the operating
ambient temperature for the fixed T_J(MAX) and the thermal
resistance, θ_JA. The derating curves in Figure 6 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
4.0
Four-Layer PCB
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 6. Derating Curve of Maximum Power Dissipation
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DS6550A-00 September 2016