XRP7714 Datasheet, PDF (25/27 Pages) Exar Corporation – Quad Channel Digital PWM Step Down Controller

XRP7714
Quad Channel Digital PWM Step Down Controller
When trying to optimize control loop bandwidth, particularly at switching frequencies below
600kHz, an effective ESR in the range of 7 to 20mohm can help significantly. The Digital Power
Studio design tool is used to verify what will work best in your application.
• Input Capacitor Selection
Select the input capacitor for Voltage, Capacitance, ripple current, ESR and ESL. Voltage rating is
nominally selected to be at least twice the input voltage. The RMS value of input capacitor current,
assuming a low inductor ripple current, can be approximated as:
1
Where:
Iin is the RMS input current
Iout is the DC output current
D is the duty cycle
In general, the total input voltage ripple should be kept below 1.5% of VIN. The input voltage
ripple also has two major components: the voltage drop on the main capacitor ΔV_Cin and the
voltage drop due to ESR - ΔV_ESR . The contribution to Input voltage ripple by each term can be
calculated from:
Δ V_Cin
=
I _outV_out (V_in − V_out )
f
s
C
V²
in in
ΔV_ESR = ESR ⋅ (I_out + 0.5I_rip )
Total input voltage ripple is the sum of the above:
ΔV_Tot = ΔV_Cin + ΔV_ESR
Power MOSFETs Selection
Selecting MOSFETs with lower Rdson reduces conduction losses at the expense of increased
switching losses. A simplified expression for conduction losses is given by:
P_cond
=
I2
out
⋅
R_dson
⋅
V_out
V_in
MOSFET’s junction temperature can be estimated from:
T _j = 2 P_cond R_thja + T_ambient
This assumes that the switching loss is the same as the conduction loss. R_thja is the total MOSFET
thermal resistance from junction to ambient.
© 2010 Exar Corporation
25/27
Rev. 1.1.3