I am not sure if it is too late. The user manual for lpc43xx states
12.2 General description
Remark: Before selecting the Deep-sleep mode or Power-down mode, you must select the IRC as the clock source for all output clocks through the CGU registers (see Section 13.8.1 ) and power down all PLLs.
The confusion comes because sections 12.2.3 and 12.2.4 recommended steps do not mention this.
Also need to disable semihosting from your project.
I read further, it is not that simple, especially for deep power down mode
a) Only 256 bytes of register starting from 0x4004 1000 is retained.
b) Need to wait awhile to fetch code from flash into memory
c) Optionally to switch back to core clock, adjust clock source, and ramp up the clock speed
c) Need to initialize to the state before you going to deep power down sleep (should store the value in flash before that mode)
d) Continue as usual
The solution is very specific to your application. I could not find any working example from the Internet.