MC34844_10 Datasheet, PDF (56/62 Pages) Freescale Semiconductor, Inc – 10 Channel LED Backlight Driver

COMPONENTS CALCULATION
C_Comp1
=
---------------------------2----------------------------
f_Cross × R_Comp × π × 2
C_Comp2
=
----------G-----^M------------
6.28 × F_SW
In order to improve the transient response of the boost, on
the 34844A, a resistor divider has been implemented from
the PWM pin to ground with a connection to the
compensation network. This configuration should inject a
1.0 V signal to the COMP pin and the Thevenin-equivalent
resistance of the divider is close to RCOMP, i.e. R_COMP = 6.8 k
and R_PCOMP= 27 k for a 5.0 V PWM signal.
PWM
R_PCOMP
R_COMP
C_COMP1
COMP PIN
C_COMP2
Slope Compensation
Slope Compensation can be expressed either in terms of
Ampers/Second or as Volts/Second, through the use of the
transfer resistance.
The following formula express the Slope Compensation in
terms of V/μs:
V_SLOPE = ⁽---V-----o---u---t---–---L--V---_×--i--n-2--⁾----^×----C-----S---G---
Where “L” is in μH
In order to have this slope compensation, the following
resistor should be set.
R_SLOPE = _V-----S-3--_L-3--_O--^×--_P-1--_E-0---³_×-----5-
Variable Definition
D= Boost Duty Cycle
V_OUT= Output Voltage
V_D= Diode Forward Voltage
V_IN= Input Voltage
V_SW= V_DROP of Internal Switch
ΔV_OUT= Output Voltage Ripple Ratio
IIN_AVG= Average Input Current
I_OUT= Output Current
r=Input Current Ratio
IIN_MAX= Maximum input current
IRMS_CIN= RMS current for Input Capacitor
IRMS_COUT= RMS current for Output Capacitor
L= Inductor
R_W= Inductor winding DC Resistance
f_SW= Boost Switching Frequency
CSG= Current Sense Gain = 0.2 V/A
A_CSA= Current Sense Amplifier Gain = 9
R_SENSE= Current Sense Resistor = 22mohm
C_OUT= Output Capacitor
R_COMP= Compensation Resistor
G_M= OTA Transconductance
ESR_COUT= ESR of Output Capacitor
f_RHPZ= Right Half Plane Zero Frequency
f_CROSS= Crossover Frequency
C_COMP1= Compensation Capacitor
C_COMP2= Shunt Compensation Capacitor
V_SLOPE= Slope Compensation (V/μs)
R_SLOPE= External Resistor for Slope Compensation
34844
56
Analog Integrated Circuit Device Data
Freescale Semiconductor